Superscalar microprocessor employing away prediction structure

ABSTRACT

A superscalar microprocessor employing a way prediction structure is provided. The way prediction structure predicts a way of an associative cache in which an access will hit, and causes the data bytes from the predicted way to be conveyed as the output of the cache. The typical tag comparisons to the request address are bypassed for data byte selection, causing the access time of the associative cache to be substantially the access time of the direct-mapped way prediction array within the way prediction structure. Also included in the way prediction structure is a way prediction control unit configured to update the way prediction array when an incorrect way prediction is detected. The clock cycle of the superscalar microprocessor including the way prediction structure with its caches may be increased if the cache access time is limiting the clock cycle. Additionally, the associative cache may be retained in the high frequency superscalar microprocessor (which might otherwise employ a direct-mapped cache for access time reasons). Single clock cycle cache access to an associative data cache is maintained for high frequency operation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to the field of superscalar microprocessorsand, more particularly, to way prediction structures employed withinsuperscalar microprocessors in order to allow associative cache accessesat speeds comparable to direct-mapped cache accesses.

2. Description of the Relevant Art

Superscalar microprocessors achieve high performance by simultaneouslyexecuting multiple instructions in a clock cycle and by specifying theshortest possible clock cycle consistent with the design. As usedherein, the term "clock cycle" refers to an interval of time duringwhich the pipeline stages of a microprocessor perform their intendedfunctions. At the end of a clock cycle, the resulting values are movedto the next pipeline stage.

Since superscalar microprocessors execute multiple instructions perclock cycle and the clock cycle is short, a high bandwidth memory systemis required to provide instructions and data to the superscalarmicroprocessor (i.e. a memory system that can provide a large number ofbytes in a short period of time). Without a high bandwidth memorysystem, the microprocessor would spend a large number of clock cycleswaiting for instructions or data to be provided, then would execute thereceived instructions and/or the instructions dependent upon thereceived data in a relatively small number of clock cycles. Overallperformance would be degraded by the large number of idle clock cycles.However, superscalar microprocessors are ordinarily configured intocomputer systems with a large main memory composed of dynamic randomaccess memory (DRAM) cells. DRAM cells are characterized by access timeswhich are significantly longer than the clock cycle of modernsuperscalar microprocessors. Also, DRAM cells typically provide arelatively narrow output bus to convey the stored bytes to thesuperscalar microprocessor. Therefore, DRAM cells provide a memorysystem that provides a relatively small number of bytes in a relativelylong period of time, and do not form a high bandwidth memory system.

Because superscalar microprocessors are typically not configured into acomputer system with a memory system having sufficient bandwidth tocontinuously provide instructions and data, superscalar microprocessorsare often configured with caches. Caches are storage devices containingmultiple blocks of storage locations, configured on the same siliconsubstrate as the microprocessor or coupled nearby. The blocks of storagelocations are used to hold previously fetched instruction or data bytes.The bytes can be transferred from the cache to the destination (aregister or an instruction processing pipeline) quickly; commonly one ortwo clock cycles are required as opposed to a large number of clockcycles to transfer bytes from a DRAM main memory.

Caches may be organized into an "associative" structure (also referredto as "set associative"). In an associative structure, the blocks ofstorage locations are accessed as a two-dimensional array having rows(often referred to as "sets") and columns (often referred to as "ways").When a cache is searched for bytes residing at an address, a number ofbits from the address are used as an "index" into the cache. The indexselects a particular set within the two-dimensional array, and thereforethe number of address bits required for the index is determined by thenumber of sets configured into the cache. The act of selecting a set viaan index is referred to as "indexing". The addresses associated withbytes stored in the multiple blocks of a set are examined to determineif any of the addresses stored in the set match the requested address.If a match is found, the access is said to be a "hit", and the cacheprovides the associated bytes. If a match is not found, the access issaid to be a "miss". When a miss is detected, the bytes are transferredfrom the memory system into the cache. The addresses associated withbytes stored in the cache are also stored. These stored addresses arereferred to as "tags" or "tag addresses".

The blocks of memory configured into a set form the columns of the set.Each block of memory is referred to as a "way"; multiple ways comprise aset. The way is selected by providing a way value to the cache. The wayvalue is determined by examining the tags for a set and finding a matchbetween one of the tags and the requested address. A cache designed withone way per set is referred to as a "direct-mapped cache". In adirect-mapped cache, the tag must be examined to determine if an accessis a hit, but the tag examination is not required to select which bytesare transferred to the outputs of the cache. Since only an index isrequired to select bytes from a direct-mapped cache, the direct-mappedcache is a "linear array" requiring only a single value to select astorage location within it.

Both direct-mapped and associative caches are employed in high frequency(i.e. short clock cycle) superscalar microprocessors. In high frequencyapplications, set associative caches either become a clock cycle limiteror require multiple clock cycles to complete a cache access because thecomparison of tags to the request address and the subsequent selectionof data bytes to convey to the output requires more time than thedesired clock cycle time allows. Cache bandwidth (i.e. the number ofcache requests which may be made per clock cycle) decrease due tomultiple clock cycle accesses is particularly detrimental to superscalarmicroprocessors attempting to execute multiple instructions (which mayrequire cache access) per clock cycle. Although cache bandwidth decreasemay be avoided by pipelining the cache access, the latency for eachaccess is still detrimental to performance. Longer clock cycles are alsodetrimental to the performance of a microprocessor, since fewerinstructions may be executed per second.

Direct-mapped caches, which compare the selected tag to the requestaddress in parallel with conveying data bytes to the output, operate inless time than the associative cache. Unfortunately, direct-mappedcaches are associated with lower hit rates (i.e. the percentage ofaccesses that are hits) than associative caches with a similar storagecapacity. Furthermore, direct-mapped caches are more susceptible to"thrashing". Thrashing is a phenomenon that occurs when the pattern ofaddress requests presented to the cache contains several dissimilaraddresses with the same index. Dissimilar addresses are addresses thatare stored in the cache with different tags. As an illustrative example,addresses A and B may access the cache alternately and repeatedly.Address A and address B have the same index, and access a direct-mappedcache. First, address A accesses the cache and misses. The indexed cachestorage location is filled with bytes associated with address A. Next,address B accesses the cache and misses. The indexed cache storagelocation discards the bytes associated with address A and is filled withbytes associated with address B. Address A accesses the cache again, andmisses. The cache storage location discards the bytes associated withaddress B and is filled with bytes associated with address A. Anassociative cache would be able to store bytes associated with bothaddress A and address B simultaneously. A storage device having theaccess time of a direct-mapped cache with the hit rate and insensitivityto thrashing of an associative cache is desired.

SUMMARY OF THE INVENTION

The problems outlined above are in large part solved by a superscalarmicroprocessor employing a way prediction structure according to thepresent invention. The present way prediction structure includes a wayprediction array and a way prediction control unit. The way predictionstructure described herein predicts a way of an associative data cachein which an access will hit, and causes the data bytes from thepredicted way to be conveyed as the output of the data cache. Thetypical tag comparisons to the input request address are bypassed fordata byte selection. Advantageously, the access time of the associativecache is substantially the access time of the direct-mapped wayprediction array. The present associative cache and way predictionstructure enable increased performance in the superscalar microprocessorin several ways. First, the clock cycle of the superscalarmicroprocessor may be increased if the cache access time is limiting theclock cycle (as is typically the case), since the cache access time ofthe associative cache is shortened. Second, the associative cache isretained in the high frequency superscalar microprocessor (which mightotherwise employ a direct-mapped cache for access time reasons). Theadvantages of an associative cache over a direct-mapped cache, includinginsensitivity to thrashing and superior hit rate characteristics, arethereby available at higher frequencies then were previously attainable.

As a result of the shortened access time enabled by the presentinvention, single clock cycle cache access to an associative data cacheis maintained for high frequency operation. Since most operationscomplete cache access in a single cycle, the cache bandwidth (asmeasured in accesses per clock cycle) is higher than a cache in whichaccesses require multiple cycles. The increased cache bandwidth isadvantageously available for additional cache accesses. Additionally,performance may be advantageously increased by the reduced latencyassociated with performing the access in a single clock cycle. Cachebandwidth increase and latency decrease are particularly valuable tosuperscalar microprocessors in which multiple instructions (possiblyrequiring cache access) are executed in a given clock cycle.

Broadly speaking, the present invention contemplates a superscalarmicroprocessor comprising a data cache including a way prediction array,a way prediction control unit, and a second plurality of storagelocations. The way prediction array includes a plurality of storagelocations wherein each one of the plurality of storage locations isconfigured to store a way value. The way prediction array is configuredto receive at least one request address. The way prediction control unitis coupled to the way prediction array and is configured to update theway prediction array. The second plurality of storage locations isconfigured to store data bytes and to receive the request address. Thesecond plurality of storage locations is coupled to the way predictionarray, and one of the second plurality of storage locations is selectedby the way value to convey data bytes upon an output bus.

The present invention further contemplates a method for accessing anassociative cache with a direct-mapped access time comprising at leasttwo steps. First, the way of the associative cache that an access willhit is predicted. During a subsequent clock cycle, the prediction isvalidated.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings in which:

FIG. 1 is a block diagram of a superscalar microprocessor including adata cache having a way prediction structure according to the presentinvention.

FIG. 2 is a block diagram of the data cache shown in FIG. 1, depictingseveral portions of one embodiment of the way prediction structureaccording to the present invention.

FIG. 3 is a timing diagram depicting a sequence of two cache accesses inwhich the way prediction is correct.

FIG. 4 is a timing diagram depicting a sequence of two cache accesses inwhich the way prediction for the first cache access is incorrect.

FIGS. 5-67 depict a superscalar microprocessor.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE INVENTION

Referring next to FIG. 1, a block diagram of a superscalarmicroprocessor 200 including a data cache 224 in accordance with thepresent invention is shown. As illustrated in the embodiment of FIG. 1,superscalar microprocessor 200 includes a prefetch/predecode unit 202and a branch prediction unit 220 coupled to an instruction cache 204.Instruction alignment unit 206 is coupled between instruction cache 204and a plurality of decode units 208A-208D (referred to collectively asdecode units 208). Each decode unit 208A-208D is coupled to respectivereservation station units 210A-210D (referred to collectively asreservation stations 210), and each reservation station 210A-210D iscoupled to a respective functional unit 212A-212D (referred tocollectively as functional units 212). Decode units 208, reservationstations 210, and functional units 212 are further coupled to a reorderbuffer 216, a register file 218 and a load/store unit 222. A data cache224 is finally shown coupled to load/store unit 222, and an MROM unit209 is shown coupled to instruction alignment unit 206.

Generally speaking, instruction cache 204 is a high speed cache memoryprovided to temporarily store instructions prior to their dispatch todecode units 208. In one embodiment, instruction cache 204 is configuredto cache up to 32 kilobytes of instruction code organized in lines of 16bytes each (where each byte consists of 8 bits). During operation,instruction code is provided to instruction cache 204 by prefetchingcode from a main memory (not shown) through prefetch/predecode unit 202.It is noted that instruction cache 204 could be implemented in aset-associative, a fully-associative, or a direct-mapped configuration.

Prefetch/predecode unit 202 is provided to prefetch instruction codefrom the main memory for storage within instruction cache 204. In oneembodiment, prefetch/predecode unit 202 is configured to burst 64-bitwide code from the main memory into instruction cache 204. It isunderstood that a variety of specific code prefetching techniques andalgorithms may be employed by prefetch/predecode unit 202.

As prefetch/predecode unit 202 fetches instructions from the mainmemory, it generates three predecode bits associated with each byte ofinstruction code: a start bit, an end bit, and a "functional" bit. Thepredecode bits form tags indicative of the boundaries of eachinstruction. The predecode tags may also convey additional informationsuch as whether a given instruction can be decoded directly by decodeunits 208 or whether the instruction must be executed by invoking amicrocode procedure controlled by MROM unit 209, as will be described ingreater detail below.

Table 1 indicates one encoding of the predecode tags. As indicatedwithin the table, if a given byte is the first byte of an instruction,the start bit for that byte is set. If the byte is the last byte of aninstruction, the end bit for that byte is set. If a particularinstruction cannot be directly decoded by the decode units 208, thefunctional bit associated with the first byte of the instruction is set.On the other hand, if the instruction can be directly decoded by thedecode units 208, the functional bit associated with the first byte ofthe instruction is cleared. The functional bit for the second byte of aparticular instruction is cleared if the opcode is the first byte, andis set if the opcode is the second byte. It is noted that in situationswhere the opcode is the second byte, the first byte is a prefix byte.The functional bit values for instruction byte numbers 3-8 indicatewhether the byte is a MODRM or an SIB byte, or whether the byte containsdisplacement or immediate data.

                  TABLE 1                                                         ______________________________________                                        Encoding of Start, End and Functional Bits                                    Instr.  Start    End    Functional                                            Byte    Bit      Bit    Bit                                                   Number  Value    Value  Value    Meaning                                      ______________________________________                                        1       1        X      0        Fast decode                                  1       1        X      1        MROM instr.                                  2       0        X      0        Opcode is first                                                               byte                                         2       0        X      1        Opcode is this                                                                byte, first                                                                   byte is prefix                               3-8     0        X      0        Mod R/M or                                                                    SIB byte                                     3-8     0        X      1        Displacement or                                                               immediate data;                                                               the second                                                                    functional bit                                                                set in bytes                                                                  3-8 indicates                                                                 immediate data                               1-8     X        0      X        Not last byte                                                                 of instruction                               1-8     X        1      X        Last byte of                                                                  instruction                                  ______________________________________                                    

As stated previously, in one embodiment certain instructions within thex86 instruction set may be directly decoded by decode unit 208. Theseinstructions are referred to as "fast path" instructions. The remaininginstructions of the x86 instruction set are referred to as "MROMinstructions". MROM instructions are executed by invoking MROM unit 209.More specifically, when an MROM instruction is encountered, MROM unit209 parses and serializes the instruction into a subset of defined fastpath instructions to effectuate a desired operation. A listing ofexemplary x86 instructions categorized as fast path instructions as wellas a description of the manner of handling both fast path and MROMinstructions will be provided further below.

Instruction alignment unit 206 is provided to channel variable bytelength instructions from instruction cache 204 to fixed issue positionsformed by decode units 208A-208D. Instruction alignment unit 206independently and in parallel selects instructions from three groups ofinstruction bytes provided by instruction cache 204 and arranges thesebytes into three groups of preliminary issue positions. Each group ofissue positions is associated with one of the three groups ofinstruction bytes. The preliminary issue positions are then mergedtogether to form the final issue positions, each of which is coupled toone of decode units 208.

Before proceeding with a detailed description of the data cache 224,general aspects regarding other subsystems employed within the exemplarysuperscalar microprocessor 200 of FIG. 1 will be described. For theembodiment of FIG. 1, each of the decode units 208 includes decodingcircuitry for decoding the predetermined fast path instructions referredto above. In addition, each decode unit 208A-208D routes displacementand immediate data to a corresponding reservation station unit210A-210D. Output signals from the decode units 208 include bit-encodedexecution instructions for the functional units 212 as well as operandaddress information, immediate data and/or displacement data.

The superscalar microprocessor of FIG. 1 supports out of orderexecution, and thus employs reorder buffer 216 to keep track of theoriginal program sequence for register read and write operations, toimplement register renaming, to allow for speculative instructionexecution and branch misprediction recovery, and to facilitate preciseexceptions. As will be appreciated by those of skill in the art, atemporary storage location within reorder buffer 216 is reserved upondecode of an instruction that involves the update of a register tothereby store speculative register states. Reorder buffer 216 may beimplemented in a first-in-first-out configuration wherein speculativeresults move to the "bottom" of the buffer as they are validated andwritten to the register file, thus making room for new entries at the"top" of the buffer. Other specific configurations of reorder buffer 216are also possible, as will be described further below. If a branchprediction is incorrect, the results of speculatively-executedinstructions along the mispredicted path can be invalidated in thebuffer before they are written to register file 218.

The bit-encoded execution instructions and immediate data provided atthe outputs of decode units 208A-208D are routed directly to respectivereservation station units 210A-210D. In one embodiment, each reservationstation unit 210A-210D is capable of holding instruction information(i.e., bit encoded execution bits as well as operand values, operandtags and/or immediate data) for up to three pending instructionsawaiting issue to the corresponding functional unit. It is noted thatfor the embodiment of FIG. 1, each decode unit 208A-208D is associatedwith a dedicated reservation station unit 210A-210D, and that eachreservation station unit 210A-210D is similarly associated with adedicated functional unit 212A-212D. Accordingly, four dedicated "issuepositions" are formed by decode units 208, reservation station units 210and functional units 212. Instructions aligned and dispatched to issueposition 0 through decode unit 208A are passed to reservation stationunit 210A and subsequently to functional unit 212A for execution.Similarly, instructions aligned and dispatched to decode unit 208B arepassed to reservation station unit 210B and into functional unit 212B,and so on.

Upon decode of a particular instruction, if a required operand is aregister location, register address information is routed to reorderbuffer 216 and register file 218 simultaneously. Those of skill in theart will appreciate that the x86 register file includes eight 32 bitreal registers (i.e., typically referred to as EAX, EBX, ECX, EDX, EBP,ESI, EDI and ESP). Reorder buffer 216 contains temporary storagelocations for results which change the contents of these registers tothereby allow out of order execution. A temporary storage location ofreorder buffer 216 is reserved for each instruction which, upon decode,is determined to modify the contents of one of the real registers.Therefore, at various points during execution of a particular program,reorder buffer 216 may have one or more locations which contain thespeculatively executed contents of a given register. If following decodeof a given instruction it is determined that reorder buffer 216 has aprevious location or locations assigned to a register used as an operandin the given instruction, the reorder buffer 216 forwards to thecorresponding reservation station either: 1) the value in the mostrecently assigned location, or 2) a tag for the most recently assignedlocation if the value has not yet been produced by the functional unitthat will eventually execute the previous instruction. If the reorderbuffer has a location reserved for a given register, the operand value(or tag) is provided from reorder buffer 216 rather than from registerfile 218. If there is no location reserved for a required register inreorder buffer 216, the value is taken directly from register file 218.If the operand corresponds to a memory location, the operand value isprovided to the reservation station unit through load/store unit 222.

Details regarding suitable reorder buffer implementations may be foundwithin the publication "Superscalar Microprocessor Design" by MikeJohnson, Prentice-Hall, Englewood Cliffs, N.J., 1991, and within theco-pending, commonly assigned patent application entitled "HighPerformance Superscalar Microprocessor", Serial No. 08/146,382, filedOct. 29, 1993 by Witt, et al. These documents are incorporated herein byreference in their entirety.

Reservation station units 210A-210D are provided to temporarily storeinstruction information to be speculatively executed by thecorresponding functional units 212A-212D. As stated previously, eachreservation station unit 210A-210D may store instruction information forup to three pending instructions. Each of the four reservation stations210A-210D contain locations to store bit-encoded execution instructionsto be speculatively executed by the corresponding functional unit andthe values of operands. If a particular operand is not available, a tagfor that operand is provided from reorder buffer 216 and is storedwithin the corresponding reservation station until the result has beengenerated (i.e., by completion of the execution of a previousinstruction). It is noted that when an instruction is executed by one ofthe functional units 212A-212D, the result of that instruction is passeddirectly to any reservation station units 210A-210D that are waiting forthat result at the same time the result is passed to update reorderbuffer 216 (this technique is commonly referred to as "resultforwarding"). Instructions are issued to functional units for executionafter the values of any required operand(s) are made available. That is,if an operand associated with a pending instruction within one of thereservation station units 210A-210D has been tagged with a location of aprevious result value within reorder buffer 216 which corresponds to aninstruction which modifies the required operand, the instruction is notissued to the corresponding functional unit 212 until the operand resultfor the previous instruction has been obtained. Accordingly, the orderin which instructions are executed may not be the same as the order ofthe original program instruction sequence. Reorder buffer 216 ensuresthat data coherency is maintained in situations where read-after-writedependencies occur.

In one embodiment, each of the functional units 212 is configured toperform integer arithmetic operations of addition and subtraction, aswell as shifts, rotates, logical operations, and branch operations. Itis noted that a floating point unit (not shown) may also be employed toaccommodate floating point operations.

Each of the functional units 212 also provides information regarding theexecution of conditional branch instructions to the branch predictionunit 220. If a branch prediction was incorrect, branch prediction unit220 flushes instructions subsequent to the mispredicted branch that haveentered the instruction processing pipeline, and causesprefetch/predecode unit 202 to fetch the required instructions frominstruction cache 204 or main memory. It is noted that in suchsituations, results of instructions in the original program sequencewhich occur after the mispredicted branch instruction are discarded,including those which were speculatively executed and temporarily storedin load/store unit 222 and reorder buffer 216. Exemplary configurationsof suitable branch prediction mechanisms are well known.

Results produced by functional units 212 are sent to the reorder buffer216 if a register value is being updated, and to the load/store unit 222if the contents of a memory location is changed. If the result is to bestored in a register, the reorder buffer 216 stores the result in thelocation reserved for the value of the register when the instruction wasdecoded. As stated previously, results are also broadcast to reservationstation units 210A-210D where pending instructions may be waiting forthe results of previous instruction executions to obtain the requiredoperand values.

Generally speaking, load/store unit 222 provides an interface betweenfunctional units 212A-212D and data cache 224. In one embodiment,load/store unit 222 is configured with a load/store buffer with eightstorage locations for data and address information for pending loads orstores. Decode units 208 arbitrate for access to the load/store unit222. When the buffer is full, a decode unit must wait until theload/store unit 222 has room for the pending load or store requestinformation. The load/store unit 222 also performs dependency checkingfor load instructions against pending store instructions to ensure thatdata coherency is maintained.

Data cache 224 is a high speed cache memory provided to temporarilystore data being transferred between load/store unit 222 and the mainmemory subsystem. In one embodiment, data cache 224 has a capacity ofstoring up to sixteen kilobytes of data. It is understood that datacache 224 may be implemented in a variety of specific memoryconfigurations, including a set associative configuration.

Turning next to FIG. 2, a block diagram of data cache 224 is shownincluding a way prediction structure according to the present invention.For simplicity, only those elements of data cache 224 pertaining to thepresent invention are shown. Generally speaking, the way predictionstructure predicts a way of data cache 224 in which a request addressmay hit, and causes the predicted way to be selected as the output databytes of data cache 224. Therefore, the tag comparisons typicallyrequired by an associative data cache are bypassed in performing the wayselection. The data may be available at the output of the data cacheafter a time delay from the start of the access similar to the timedelay of a direct-mapped cache. If the way prediction is correct, singlecycle access of an associative cache is achieved, and the clock cyclemay be consistent with a direct-mapped cache access. If the wayprediction is incorrect but the request address hits in another way,then a subsequent clock cycle is used to fetch the correct data and toupdate the way prediction to the hitting way. If the request address isa miss, then the way prediction is left unchanged and the miss isprocessed.

Each clock cycle, load/store unit 222 may convey one or more cacheaccess requests on an address request bus 250 to data cache 224. In oneembodiment, load/store unit 222 is configured to convey up to tworequests per clock cycle. Each request includes an address and requestidentification information including an indication of whether therequest is associated with a load or a store instruction. Addressrequest bus 250 is coupled to input ports on a way prediction array 251,a cache tag array 252, and a cache data array 253. Way prediction array251 is configured to store way predictions for data cache 224. Cache tagarray 252 stores tags for the data stored in data cache 224. The databytes are stored in cache data array 253. A cache control unit 254 iscoupled to cache tag array 252 and cache data array 253. Cache controlunit 254 is configured to provide cache control functions, such asmanaging the replacement algorithm for data cache 224 and creating byteenables for store cache accesses.

As shown in FIG. 2, cache tag array 252 and cache data array 253 areN-way set associative. Each clock cycle, N sets of data bytes areconveyed from output ports of cache data array 253 on a set of way databuses 255A-255N (collectively referred to as way data buses 255). Eachof way data buses 255 conveys data bytes from a particular way indexedby addresses conveyed on address request bus 250. One set of data bytesis conveyed on each of way data buses 255 for each address which may beconveyed on address request bus 250 in a given clock cycle. In oneembodiment, data cache 224 is eight-way set associative.

Way data buses 255 are coupled to an N-to-One way selection block 256.In one embodiment, way selection block 256 is a set of multiplexors (onefor each address which may be conveyed on address request bus 250). Eachmultiplexor is coupled to way data buses 255 and to a way prediction bus257 from way prediction array 251. A way prediction for each requestedaddress is conveyed from output ports of way prediction array 251 on wayprediction bus 257 to way selection block 256. The way prediction isindicative of one of the N ways of data cache 224. In one embodiment, away prediction comprises a decoded value wherein a particular way isindicated through the setting of a particular bit within the decodedvalue. In another embodiment, the value is encoded as a number betweenzero and N-1 wherein each value corresponds to the selected way. Thedata bytes associated with the predicted way for each request addressare conveyed on a data bus 258 to load/store unit 222 and reservationstations 210.

Way prediction array 251 is configured to store way predictions for datacache 224. While cache tag array 252 and cache data array 253 areconfigured similarly with respect to numbers of ways and numbers ofsets, way prediction array may be configured differently. In particular,more than one way prediction for a particular set of cache tag array 252and cache data array 253 may be stored within way prediction array 251.For example, if way prediction array 251 is configured with twice asmany sets as cache tag array 252 and cache data array 253, then two waypredictions will be stored within way prediction array 251 for each setof cache tag array 252 and cache data array 253. The number of bits ofthe request address used to index way prediction array 251 would be onelarger than the number used to index cache tag array 252 and cache dataarray 253 for this example, and the extra bit determines which of thetwo stored way predictions is selected as the way prediction for thatrequest address. It is noted that the likelihood of storing a correctway prediction for a given address is increased by the number of waypredictions stored for the indexed set of cache tag array 252 and cachedata array 253. Performance may therefore be increased by using a wayprediction array having more than one way prediction per set of cachetag array 252 and cache data array 253. In one embodiment, wayprediction array 251 is a linear array of 512 way predictions, and eachway prediction is a decoded value as described above. Generallyspeaking, a preferred embodiment of way prediction array 251 is a lineararray comprising a number of way prediction storage locations equal innumber to the number of sets multiplied by the number of ways in datacache array 253.

While a request address is indexing into cache tag array 252 and cachedata array 253, the request address is also indexing into way predictionarray 251. Way prediction array 251 conveys the indexed way predictionon way prediction bus 257, causing the data bytes associated with thepredicted way to be conveyed on data bus 258. Therefore, the time delayfrom the beginning of an access to data being provided by data cache 224is characterized by the time delay of a direct-mapped cache access (wayprediction array 251) followed by a small delay associated with theselection of output bytes using the predicted way. Such a time delay issubstantially similar to the access time of a direct-mapped cache.Advantageously, associative data cache 224 is characterized by adirect-mapped cache access time.

Additionally, way prediction bus 257 is coupled to a tag comparisonblock 259. Tag comparison block 259 embodies logic circuits configuredto determine whether or not the request address hits in the predictedway, hits in an unpredicted way, or misses. Tag comparison block 259compares the indexed tags (as conveyed on a set of way tag buses264A-264N similar to way data buses 255) to the request address, thendetermines if the actual hit way from the tag comparisons matches thepredicted way from way prediction array 251. Since the access to cachetag array 252 and the tag comparisons that follow may not complete in agiven clock cycle, tag comparison block 259 produces signals early in aclock cycle subsequent to the clock cycle in which the associatedaddresses access data cache 224.

If a hit in the predicted way is detected for a load request, a signalis asserted on a hit predicted way bus 260. Hit predicted way bus 260 iscoupled between tag comparison block 259 and load/store unit 222 andreservation stations 210. A signal associated with each possible requestaddress is conveyed on hit predicted way bus 260, and both load/storeunit 222 and reservation stations 210 are configured to associate eachsignal with the appropriate address request from the previous clockcycle. A hit in the predicted way indicates that the data conveyed ondata bus 258 in the previous clock cycle is correct. Reservationstations 210 may then discard any instructions which received the dataand executed in the current clock cycle, and load/store unit 222 maydiscard the load instruction associated with the requested address.Additionally, a functional unit 212 which received the incorrect datasends a cancel indication along with the value it calculates using theincorrect data during this clock cycle such that reorder buffer 216 doesnot record the incorrect value and such that reservation stations 210ignore the forwarded valued.

If a hit in an unpredicted way is detected for a load request, acorresponding signal is asserted on a hit unpredicted way bus 261 whichis coupled to load/store unit 222 and reservation stations 210. A hit inan unpredicted way causes load/store unit 222 to cancel a requestaddress conveyed on the portion of request address bus 250 upon whichthe unpredicted way hit address was conveyed in the previous clockcycle. Data cache 224 uses the idle port to convey the correct data ondata bus 258. The way prediction is corrected while the corrected datais conveyed on data bus 258. The way prediction is corrected via a wayprediction control unit 265. Way prediction control unit 265 receives ahit way bus 263 and hit unpredicted way bus 261. When a hit in anunpredicted way signal is asserted, the way in which the hit occurs isconveyed on hit way bus 263. Way prediction control unit 265 conveys thehitting way to a write port on way prediction array 251, and signals wayprediction array 251 to write the value into the location indexed by theassociated address stored by data cache 224 from the previous clockcycle. Thus, a hit in an unpredicted way is penalized only a clockcycle. Additionally, reservation stations 210 detect the hit in theunpredicted way and cause the data forwarded in the previous clock cyclewith respect to the associated address to be invalidated. If aninstruction which received the incorrect data as an operand is currentlybeing executed by the respective functional unit 212, reservationsstations 210 assert a signal to other reservation stations and reorderbuffer 216 indicative of the incorrect data so that the otherreservation stations and reorder buffer 216 ignore any results fromexecuting an instruction with the incorrect data.

If a miss is detected, a corresponding signal is asserted on a miss bus262 which is coupled to load/store unit 222 and reservation stations210. Load/store unit 222 records the miss for later miss processing, andreservation stations 210 perform similar actions to the case of a hit inan unpredicted way described above. Way prediction array 251 is notmodified for this case, since there is no correct predicted way for acache miss. In another embodiment, miss bus 262 is not included and amiss is detected via the absence of appropriate signal assertions uponhit predicted way bus 260 and hit unpredicted way bus 261.

It is noted that the above discussion applies to the effects of wayprediction on load accesses to data cache 224. Store requests may beperformed according to the disclosure in a commonly assigned, co-pendingpatent application entitled: "A Data Cache Capable of Performing StoreAccesses in a Single Clock Cycle", filed concurrently herewith by Witt,et al. The disclosure of this patent application is incorporated hereinby reference in its entirety. Otherwise, store requests are performed astwo accesses in subsequent clock cycles. During a first access, the wayof data cache 224 storing bytes associated with the store address isdetermined. The way prediction structure returns the predicted way.During a second access, the store is performed to the way determined inthe first access. If the predicted way is found to be incorrect, thesecond access is cancelled and the correct way is recorded by load/storeunit 222 during a repeat of the first access (similar to the load accesshit in an unpredicted way, described above). During a clock cyclesubsequent to the repeated first access, the second access is performed.Cache control unit 254 is configured to generate byte enables for datacache 224 such that only the bytes affected by the store request areoverwritten within the selected storage location. Other bytes are leftunchanged.

In one embodiment, way prediction array 251 is initialized at power upof microprocessor 200 such that way zero is the way prediction stored ineach entry. Subsequent accesses to data cache 224 cause the variousstored way predictions to change as described above.

Turning now to FIG. 3, a timing diagram showing the operation of the wayprediction structure for two successive correct way predictions isshown. Three clock cycles are shown, as indicated by the CLK1, CLK2, andCLK3 labels shown in FIG. 3. During CLK1, a request for an address "A"is conveyed from load/store unit 222 to data cache 224, as indicated byblock 300. Near the end of CLK1, data from the predicted way isforwarded on data bus 258, as indicated by arrow 301. Early in CLK2, tagcomparison block 259 determines that address A hits in the predictedway, and therefore the data conveyed on data bus 258 during CLK1 iscorrect. The corresponding signal on hit predicted way bus 260 isasserted, as indicated by arrow 302. Load/store unit 222 conveys arequest for an address "B" in CLK2, as indicated by block 303. Data fromthe predicted way is forwarded on data bus 258 as indicated by arrow304. During CLK3, tag comparison block 259 determines that address Bhits in the way predicted during CLK2. Once again, the correspondingsignal on hit predicted way bus 260 is asserted (as indicated by arrow305). As can be seen from FIG. 3, the way prediction structure enablessingle clock cycle access to associative data cache 224 with accesstimes characteristic of direct-mapped caches as long as the wayprediction is correct.

Turning next to FIG. 4, a timing diagram showing two consecutiveaccesses in which the first access hits in an unpredicted way is shown.As with FIG. 3, consecutive clock cycles are depicted on the timingdiagram. The clock cycles are labeled CLK1, CLK2, CLK3, and CLK4. DuringCLK1, load/store unit 222 conveys a request for an address "A" to datacache 224, as indicated by block 400. Near the end of CLK1, data fromthe predicted way is forwarded on data bus 258, as indicated by arrow401. Early in CLK2, tag comparison block 259 detects a hit in one of theunpredicted ways of data cache 224, and therefore the data conveyed ondata bus 258 during CLK1 is incorrect. The corresponding signal on hitunpredicted way bus 261 is asserted, as indicated by arrow 402.Load/store unit 222 begins conveying a request for an address "B" onaddress request bus 250, but cancels that request due to the hit in theunpredicted way, as indicated by block 403. Address A is subsequentlyconveyed on address request bus 250 (shown as block 404). Near the endof CLK2, the way prediction array is updated with the corrected wayprediction for address A and the correct data is forwarded on data bus258 (as indicated by arrow 405). During CLK3, a hit in the predicted waywill be signaled since the corrected way is conveyed on way predictionbus 257, as indicated by arrow 406. Therefore, one extra clock cycle isused to provide data which hits in an unpredicted way.

Also during CLK3, load/store unit 222 conveys the request for address Bthat it attempted to convey in CLK2 (as indicated by block 407). Since ahit in the predicted way is signaled by data cache 224, the request foraddress B remains active in CLK3 and data from the predicted way foraddress B is forwarded (as indicated by arrow 408). In CLK4, arrow 409indicates that address B hits in the predicted way and so data forwardedduring CLK3 with respect to address B data is correct. Another addressrequest could be conveyed by load/store unit 222 in CLK4 and it wouldreceive data forwarded from its predicted way near the end of CLK4.

It is noted that although the foregoing discussion describes the wayprediction structure with respect to a data cache, a similar structureis contemplated for instruction caches as well.

Turning next to FIGS. 5-66, details regarding various aspects of anotherembodiment of a superscalar microprocessor are next considered. FIG. 5is a block diagram of a processor 500 including an instruction cache 502coupled to a prefetch/predecode unit 504, to a branch prediction unit506, and to an instruction alignment unit 508. A set 510 of decode unitsis further coupled to instruction alignment unit 508, and a set 512 ofreservation station/functional units is coupled to a load/store unit 514and to a reorder buffer 516. A register file unit 518 is finally showncoupled to reorder buffer 516, and a data cache 522 is shown coupled toload/store unit 514.

Processor 500 limits the addressing mechanism used in the x86 to achieveboth regular simple form of addressing as well as high clock frequencyexecution. It also targets 32-bit O/S and applications. Specifically,32-bit flat addressing is employed where all the segment registers aremapped to all 4 GB of physical memory. the starting address being0000-0000 hex and their limit address being FFFF hex. The setting ofthis condition will be detected within processor 500 as one of theconditions to allow the collection of accelerated data paths andinstructions to be enabled. The absence of this condition of 32-bit flataddressing will cause a serialization condition on instruction issue anda trapping to MROM space.

Another method to ensure that a relatively high clock frequency may beaccommodated is to limit the number of memory address calculationschemes to those that are simple to decode and can be decoded within afew bytes. We are also interested in supporting addressing that fitsinto our other goals, i.e., regular instruction decoding.

As a result, the x86 instruction types that are supported for load/storeoperations are:

    ______________________________________                                                push  implied ESP - 4!                                                        pop  implied ESP + 4!                                                         call  implied ESP + 8!                                                        ret (implied ESP - 8!                                                         load  base + 8-bit displacement!                                              store  base + 8-bit displacement!                                             oper.  EBP + 8-bit displacement!                                              oper. (EAX + 8-bit displacement!                                      ______________________________________                                    

The block diagram of FIG. 6 shows the pipeline for calculatingaddressing within processor 500. It is noted that base +8/32 bitdisplacement takes 1 cycle, where using an index register takes 1 morecycle of delay in calculating the address. More complicated addressingthan these requires invoking an MROM routine to execute.

An exemplary listing of the instruction sub-set supported by processor500 as fast path instructions is provided below. All other x86instructions will be executed as micro-ROM sequences of fast pathinstructions or extensions to fast path instructions.

The standard x86 instruction set is very limited in the number ofregisters it provides. Most RISC processors have 32 or greater generalpurpose registers, and many important variables can be held during andacross procedures or processes during normal execution of routines.Because there are so few registers in the x86 architecture and most arenot general purpose, a large percentage of operations are moves to andfrom memory. RISC architectures also incorporate 3 operand addressing toprevent moves from occurring of register values that are desired to besaved instead of overwritten.

The x86 instruction set uses a set of registers that can trace itshistory back to the 8080. Consequently there are few registers, manyside effects, and sub-registers within registers. This is because whenmoving to 16-bit, or 32-bit operands, mode bits were added and thelengths of the registers were extended instead of expanding the size ofthe register file. Modern compiler technology can make use of largeregister sets and have a much smaller percentage of loads and stores.The effect of these same compilers is to have a much larger percentageof loads and stores when compiling to the x86. The actual x86 registersare often relegated to temporary registers for a few clock cycles whilethe real operation destinations are in memory.

FIG. 7 shows a programmer's view of the x86 register file. One notesfrom this organization that there are only 8 registers. and few aregeneral purpose. The first four registers, EAX, EDX, ECX, and EBX, haveoperand sizes of 8, 16, or 32-bits depending on the mode of theprocessor or instruction. The final 4 registers were added with the 8086and extended with the 386. Because there are so few real registers, theytend to act as holding positions for the passing of variables to andfrom memory.

The important thing to note is that when executing x86 instructions, onemust be able to efficiently handle 8, 16, and 32-bit operands. If one istrying to execute multiple x86 instructions in parallel, it is notenough to simply multi-port the register file. This is because there aretoo few registers and all important program variables must be held inmemory on the stack or in a fixed location.

RISC designs employ regular instruction decoding along naturalboundaries to achieve very high clock frequencies and also with a smallnumber of pipeline stages even for very wide issue processors. This ispossible because finding a large number of instructions and theiropcodes is relatively straightforward, since they are always at fixedboundaries.

As stated previously, this is much more difficult in an x86 processorwhere there are variable byte instruction formats, as well as prefixbytes and SIB bytes that can effect the length and addressing/data typesof the original opcode.

Processor 500 employs hardware to detect and send simple instructions tofixed issue positions, where the range of bytes that a particular issueposition can use is limited. This may be compensated for by adding manyissue positions that each instruction cache line can assume in parallel.

Once the instructions are aligned to a particular issue position, thenet amount of hardware required to decode common instructions is notsignificantly greater than that of a RISC processor, allowing equivalentclock frequencies to be achieved. Processor 500 achieves high frequency,wide issue, and limited pipeline depth by limiting the instructionsexecuted at high frequency to a sub-set of the x86 instructions underthe conditions of 32-bit flat addressing.

The results of executing instructions are returned to the correspondingentry in the reorder buffer. If a store, the store is held inspeculative state in front of the data cache in a store buffer, fromwhich point it can be speculatively forwarded from. The reorder bufferthen can either cancel this store or allow it to writeback to the datacache when the line is retired.

The following set of instructions probably comprise 90% of thedynamically executed code for 32-bit applications:

8/32-bit operations

move reg/reg reg/mem

arithmetic operations reg/mem reg/reg logical operations reg/reg reg/mempush

logical operations reg/reg reg/mem

push

pop

call/return

load effective address

jump cc

jump unconditional

16-bit operations

prefix/move reg/reg

prefix/move reg/mem

prefix/arithmetic operations reg/reg, reg/mem

prefix/logical operations reg/reg reg/mem

prefix/push

prefix/pop

When executing 32-bit code under flat addressing, these instructionsalmost always fall within 1-8 bytes in length, which is in the samerough range of the aligned, accelerated fast path instructions.

Accelerated instructions are defined as fast-path instructions between 1and 8 bytes in length. It noted that it is possible that the start/endpositions predecoded reflect multiple x86 instructions, for instance 2or 3 pushes that are predecoded in a row may be treated as oneaccelerated instruction that consumes 3 bytes.

When a cache line is fetched from the instruction cache, it moves intoan instruction alignment unit which looks for start bytes within narrowranges. The instruction alignment unit uses the positions of the startbytes of the instructions to dispatch the instructions to four issuepositions. Instructions are dispatched such that each issue positionaccepts the first valid start byte within its range along withsubsequent bytes.

A multiplexer in each decoder looks for the end byte associated witheach start byte, where an end byte can be no more than seven bytes awayfrom a start byte. The mechanism to scan for a constant value in aninstruction over four bytes in length may be given an extra pipelinestage due to the amount of time potentially required.

Note that instructions included in the subset of acceleratedinstructions, and which are over four bytes in length, always have aconstant as the last 1/2/4 bytes. This constant is usually not neededuntil the instruction is issued to a functional unit, and therefore thedetermination of the constant value can be delayed in the pipeline. Theexception is an instruction requiring an eight-bit displacement for anaddress calculation. The eight-bit displacement for stack-relativeoperations is always the third byte after the start byte, so this fieldwill always be located within the same decoder as the rest of theinstruction.

It is possible that a given cache line can have more instructions toissue than can be accommodated by the four entry positions contained ineach line of the four issue reorder buffer. If this occurs, the fourissue reorder buffer allocates a second line in the buffer as theremaining instructions are dispatched. Typically, in 32-bit applicationand O/S code, the average instruction length is about three bytes. Theopcode is almost always the first two bytes, with the third byte being asib byte specifying a memory address (if included), and the fourth bytebeing a 16-bit data prefix.

The assumption in the processor 500 alignment hardware is that if theaverage instruction length is three, then four dedicated issue positionsand decoders assigned limited byte ranges should accommodate mostinstructions found within 16-byte instruction cache lines. If very densedecoding occurs (i.e., lots of one and two byte instructions), severallines are allocated in the four issue reorder buffer for the results ofinstructions contained in a few lines of the instruction cache. The factthat these more compact instructions are still issued in parallel and ata high clock frequency more than compensates for having some decoderpositions potentially idle.

Overview of the Processor 500 Instruction Cache (Icache)

This section describes the instruction cache organization, fetchingmechanism, and pre-decode information. The Processor 500 instructioncache has basic features including the ICSTORE, ICTAGV, ICNXTBLK,ICCNTL, ICALIGN, ICFPC, and ICPRED. Highlights are: the pre-decode bitsper byte of instructions are 3 bits, the branch prediction increases to2 targets, 2 different types of branch prediction techniques (bimodaland global) are implemented, the X86 instructions align to 4 fixedlength RISC-type instructions, and the pre-decode logic eliminates manyserializtion conditions. Processor 500 executes the X86 instructionsdirectly with a few instructions requiring two Rops, the BYTEQ isconfigured for fast scanning of instructions, and instructions arealigned to 4 decode units. The pre-decode data is separate in a blockcalled ICPDAT, instead of inside the ICSTORE. The pre-fetch buffers areadded to the ICSTORE to write instructions directly into the array, andthe prefixes are not modified. All branches are detected duringpre-decoding. Unconditional branches are taken during pre-decoding andaligning of instructions to the decode units. A return stack isimplemented for CALL/RETURN instructions. Way prediction is implementedto read the current block and fetch the next block because the tagcomparison and branch prediction do not resolve until the second cycle.The scanning for 4 instructions is done from ICPDAT before selected bytag comparison. Since the pre-decode data does not include theinformation for the 2-Rop instructions, the instructions must bepartially decoded for the 2-rop during prioritizing and aligning ofinstructions to decode units. The early decoding includes decoding forunconditional branches, operand addresses, flags, displacement andimmediate fields of the instruction. The CMASTER takes care of thereplacement algorithm for the Icache and sends the way associative alongwith the data to the pre-fetch buffer. This section includes signallists, timings and implementation issues for the Icache and allsub-blocks.

The Icache size is 32K bytes with 8-way set associative. The Icache islinearly addressed. The number of pipeline stages is 9. Icache will havemore than one clock cycle to read and align the instructions to thedecode units. The row decoding of index address is calculated in firsthalf of ICLK, the data, tag, pre-decode, and predicting information areread in by the end of ICLK. In the next cycle, the data are selected bythe TAGHITs and latched. The pre-decode data are scanned to generate thecontrols to the multiplexers for aligning and sending the instructionsto the decode units and MROM units. A part of the scanning logic is donein parallel with the tag comparison. The scanning and alignment ofinstructions takes two clock cycles. The decode units can start decodingin the second half of the third clock. The Icache includes away-prediction which can be done in a single clock using the ICNXTBLKtarget. The branch prediction includes bimodal and global branchprediction which takes two clock cycles. The timing from fetching,scanning, aligning, decoding, and muxing of instructions to decode unitsare shown in FIG. 8.

Throughout this documentation, a discussion of the layout organizationis included in each section. The array is organized into many sets, andeach set has its own decoder. The decoder is in the center of the set.

Signal List

SRBB(31:0)--I/O from SRB indicates the special register address for thearray or data transferring to/from the SRB.

SRB₋₋ VAL--Input from SRB indicates a special register instruction is onthe SRBB.

ICTAR₋₋ VAL--Output to SRB indicates completion of the special registerinstruction, for read the data is on the SRBB.

IRESET--Global signal used to reset ICACHE block. Clears all statemachines to Idle/Reset.

IDECJAMIC--Global signal from FIROB. Used to indicate that an interruptor trap is being taken. Effect on Icache is to clear all pre-fetch oraccess in progress, and set all state machines to Idle/Reset.

EXCEPTION--Global input from FIROB indicates that an interrupt or trapis being taken including re-synchronization. Effect on Idecode and FUsis to clear all instructions in progress.

REQTRAP--Global input from FIROB, one cycle after EXCEPTION, indicatesthat the trap is initiated with new entry point or new PC is driven.

INVBHREG--Input from FIROB to invalidate the branch holding register.The branch mis-prediction is speculative, an early branch can bemis-predicted at a later time.

CS32X16--Input from LSSEC indicates operand and address size from the Dbit of the segment descriptor of the code segment register. If set,32-bit, if clear, 16-bit.

SUPERV--Input from LSSEC indicates the supervisor mode or user mode ofthe current accessed instruction.

TR12DIC--Input from SRB indicates that all un-cached instructions mustbe fetched from the external memory.

SRBINVILV--Input from SRB to invalidate the Icache by clear all validbits.

INSTRDY--Input from BIU to indicates the valid external fetchedinstruction is on the INSB(63:0) bus.

INSTFLT--Input from BIU to indicates the valid but faulted externalfetched instruction is on the INSB(63:0) bus.

INSB(63:0)--Input from external buses for fetched instruction to theIcache.

L2₋₋ IC₋₋ ALIAS--Input from CMASTER indicates the instruction is in theIcache with different mapping. The CMASTER provides the way associativeand new supervisor bit. The LV will be set in this case.

PFREPLCOL(2:0)--Input from CMASTER indicates the way associative forwriting of the ICTAGV.

UPDFPC--Input from FIROB indicate that a new Fetch PC has been detected.This signal accompanies the FPC for the Icache to begin access the cachearrays.

FPC(31:0)--Input from FIROB as the new PC for branch correction path.

BPC(11:0)--Input from FIROB indicates the PC index and byte-pointer ofthe branch instruction which has been mis-predicted for updating theICNXTBLK. This index must be compared to the array index for exactrecovery of the global shift register.

BRNMISP--Input from the Branch execution of the FU indicates a branchmis-prediction. The Icache changes its state machine to access a new PCand clears all pending instructions.

BRNTAKEN--Input from FIROB indicate the status of the mis-prediction.This signal must be gated with UPDFPC.

BRNTAG(3:0)- Input from FIROB indicates the instruction byte forupdating the branch prediction in the ICNXTBLK.

FPCTYP--Input for FIROB indicates the type of address that is beingpassed to the Icache.

HLDISP(1:0)--Output to Idecode indicates all instructions of the first(bit 0) and/or the second (bit 1) 8-byte of the current line has beendispatched to decode units.

REFRESH2--Input from Idecode indicates current line of instructions willbe refreshed and not accept new instructions from Icache.

MROMEND--Input from MENG indicates completion of the MROM.

D0USEFL(4:0)

D0WRFL(4:0)--Output to FIROB indicates the type of flag uses/writes forthis instruction of decode unit 0:

xx1 CF--carry flag,

x1x OF--overflow flag,

1xx SF--sign, ZF--zero, PF--parity, and AF--auxiliary carry

D1USEFL(4:0)

D1WRFL(4:0)--Output to FIROB indicates the type of flag uses/writes forthis instruction of decode unit 1.

D2USEFL (4:0)

D2WRFL(4:0)--Output to FIROB indicates the type of flag uses/writes forthis instruction of decode unit 2.

D3USEFL(4:0)

D3WRFL(4:0)--Output to FIROB indicates the type of flag uses/writes forthis instruction of decode unit 3.

RD0PTR1(5:0)--Indicates the register address for operand 1 of decodeunit 0. The MROM is responsible to send bit 5:3 for the MROM register.

RD1PTR1(5:0)--Indicates the register address for operand 1 of decodeunit 1. The MROM is responsible to send bit 5:3 for the MROM register.

RD2PTR1(5:0)--Indicates the register address for operand 1 of decodeunit 2. The MROM is responsible to send bit 5:3 for the MROM register.

RD3PTR1(5:0)--Indicates the register address for operand 1 of decodeunit 3. The MROM is responsible to send bit 5:3 for the MROM register.

RD0PTR2(5:0)--Indicates register address for operand 2 of decode unit 0.The MROM is responsible to send bit 5:3 for the MROM register.

RD1PTR2(5:0)--Indicates register address for operand 2 of decode unit 1.The MROM is responsible to send bit 5:3 for the MROM register.

RD2PTR2(5:0)--Indicates register address for operand 2 of decode unit 2.The MROM is responsible to send bit 5:3 for the MROM register.

RD3PTR2(5:0)--Indicates register address for operand 2 of decode unit 3.The MROM is responsible to send bit 5:3 for the MROM register.

IDxDAT(1:0)--Output to indicates the data size information. 01--byte,10--half word, 11--word, 00--not used.

ICBTAG1(3:0)--Output to Idecode indicates the position of the firsttarget branch instruction with respect to the global shift register incase of branch mis-prediction. The branch can be taken or non-taken,branch tag must be sent with all branch instruction.

ICBTAG2(3:0)--Output to Idecode indicates the position of the secondtarget branch instruction with respect to the global shift register incase of branch mis-prediction. The branch can be taken or non-taken,branch tag must be sent with all branch instruction.

UNJMP(3:0)--Output indicates the unconditional branch instruction needsto calculate target address.

BRNTKN(3:0)--Output indicates which decode unit has a predicted takenbranch. The operand steering uses this signal to latch and sendBTADDR(31:0) to the functional unit.

BRNINST(3:0)--Output indicates which decode unit has a global branchprediction. The operand steering uses this signal to latch and sendICBTAG1(3:0) and ICBTAG2(3:0) to the functional units.

CALLDEC(3:0)--Output to FIROB indicates the CALL instruction isdetected, the return stack should be updated with the PC address ofinstruction after CALL. The information is latched for mis-predictedCALL branch.

RETDEC(3:0)--Output to FIROB indicates a RETURN instruction is detected.The information is latched for mis-predicted RETURN branch.

ICPREF(9:0)--Output to Idecode and MROM indicates the encoded prefixbyte. The two most significant bits are repeat prefixes for MROM.

IC2ROP0(3:0)--Output to decode unit 0 indicates 2-rop instruction. Bit 3indicates the first rop or second rop of the 2-rop instruction, bit 2indicates POP instruction, bit 1 indicates the MUL instruction, and bit0 indicates the SIB-byte instruction.

NODEST(3:0)--Output to FIROB indicates no destination for the first ropof the SIB-byte instruction.

DEPTAG(3:1)--Output to FIROB indicates forced dependency tag on thefirst instruction; the second rop of the SIB-byte instruction.

REFRESH2--Input from Idecode indicates current line of instructions willbe refreshed and not accept new instructions from Icache.

IB1(191:0)--Output indicates the combined instruction line fordispatching to decode units.

MROMEN--Input from MENG indicates the micro-instructions is sent toIdecode instead of the Icache.

M0USEFL(4:0)

M0WRFL(4:0)--Input from MENG indicates the type of flag used/written forthis micro-instruction of decode unit 0:

xx1 CF--carry flag,

x1x OF--overflow flag,

1xx SF--sign, ZF--zero, PF--parity, and AF--auxiliary carry

M1USEFL(4:0)

M1WRFL(4:0)--Input from MENG indicates the type of flag used/written forthis micro-instruction of decode unit 1.

M2USEFL (4:0)

M2WRFL(4:0)--Input from MENG indicates the type of flag used/written forthis micro-instruction of decode unit 2.

M3USEFL (4:0)

M3WRFL(4:0)--Input from MENG indicates the type of flag used/written forthis micro-instruction of decode unit 3.

MINS0(63:0)--Input from MENG indicates the displacement and immediatefield of micro-instruction being sent to decode 0.

MINS1(63:0)--Input from MENG indicates the displacement and immediatefield of micro-instruction being sent to decode 1.

MINS2(63:0)--Input from MENG indicates the displacement and immediatefield of micro-instruction being sent to decode 2.

MINS3(63:0)--Input from MENG indicates the displacement and immediatefield of micro-instruction being sent to decode 3.

MR0OPC(7:0)--Input from MENG to decode unit 0 indicates the opcode byte.

MR1OPC(7:0)--Input from MENG to decode unit 1 indicates the opcode byte.

MR2OPC(7:0)--Input from MENG to decode unit 2 indicates the opcode byte.

MR3OPC(7:0)--Input from MENG to decode unit 3 indicates the opcode byte.

MR0EOP(2:0)--Input from MENG to decode unit 0 indicates the extendedopcode field.

MR1EOP(2:0)--Input from MENG to decode unit 1 indicates the extendedopcode field.

MR2EOP(2:0)--Input from MENG to decode unit 2 indicates the extendedopcode field.

MR3EOP(2:0)--Input from MENG to decode unit 3 indicates the extendedopcode field.

MR0SS(1:0)--Input from MENG to decode unit 0 indicates the scale factorof the SIB byte.

MR1SS(1:0)--Input from MENG to decode unit 1 indicates the scale factorof the SIB byte.

MR2SS(1:0)--Input from MENG to decode unit 2 indicates the scale factorof the SIB byte.

MR3SS(1:0)--Input from MENG to decode unit 3 indicates the scale factorof the SIB byte.

ICMROM--Output to MENG indicates the current instruction is MROM. TheMROM instruction may take two cycles to read the IB, ICEND, and ICFUNC.

ENDINST--Input from ICPRED indicates that pre-decoding is completed forthe current instruction. The byte position of the branch instruction isfrom STARTPTR. The selected instruction from IB should be sent to decodeunit 0.

ICVALI(3:0)--Output to Idecode indicates valid instructions. NOOP isgenerated for invalid instruction.

IC0OPC(7:0)--Output to decode unit 0 indicates the opcode byte.

IC1OPC(7:0)--Output to decode unit 1 indicates the opcode byte.

IC2OPC(7:0)--Output to decode unit 2 indicates the opcode byte.

IC3OPC(7:0)--Output to decode unit 3 indicates the opcode byte.

IC0EOP(2:0)--Output to decode unit 0 indicates the extended opcodefield.

IC1EOP(2:0)--Output to decode unit 1 indicates the extended opcodefield.

IC2EOP(2:0)--Output to decode unit 2 indicates the extended opcodefield.

IC3EOP(2:0)--Output to decode unit 3indicates the extended opcode field.

IC0SS(1:0)--Output to decode unit 0indicates the scale factor of the SIBbyte.

IC1SS(1:0)--Output to decode unit 1indicates the scale factor of the SIBbyte.

IC2SS(1:0)--Output to decode unit 2indicates the scale factor of the SIBbyte.

IC3SS(1:0)--Output to decode unit 3indicates the scale factor of the SIBbyte.

DISPTR0(6:0)--Output to decode unit 0indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, bit 6:5 is thesize, and bit 4:3indicates which 8-byte block. Bit 6:5=00indicates nodisplacement.

DISPTR1(6:0)--Output to decode unit 1indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, bit 6:5 is thesize, and bit 4:3indicates which 8-byte block. Bit 6:5=00indicates nodisplacement.

DISPTR2(6:0)--Output to decode unit 2indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, bit 6:5 is thesize, and bit 4:3indicates which 8-byte block. Bit 6:5=00indicates nodisplacement.

DISPTR3(6:0)--Output to decode unit 3indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, bit 6:5 is thesize, and bit 4:3indicates which 8-byte block. Bit 6:5=00indicates nodisplacement.

IMMPTR0(4:0)--Output to decode unit 0indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, and bit4:3indicates which 8-byte block. Decoding of all opcodes is needed todetect immediate field.

IMMPTR1(4:0)--Output to decode unit 1indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, and bit4:3indicates which 8-byte block. Decoding of all opcodes is needed todetect immediate field.

IMMPTR2(4:0)--Output to decode unit 2indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, and bit4:3indicates which 8-byte block. Decoding of all opcodes is needed todetect immediate field.

IMMPTR3(4:0)--Output to decode unit 3indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, and bit4:3indicates which 8-byte block. Decoding of all opcodes is needed todetect immediate field.

CONSTn(2:0)--Output to decode unit n indicates the constant foradd/substract to ESP of the two-dispatch position instruction.

MROMEN--Input from MENG indicates the micro-instructions is sent toIdecode instead of the Icache.

IB2(191:0)--Output to decode units indicates the current line ofinstructions.

ICMROM--Output to MENG indicates the current instruction is MROM. TheMROM instruction may take two cycles to read the IB, ICEND, and ICFUNC.

ICPC1TAR--Output to Idecode indicates is ICPC1 a branch target of aprevious instruction which is a predicted taken branch instruction.

ICPC2TAR--Output to Idecode indicates is ICPC2 a branch target of aprevious instruction which is a predicted taken branch instruction.

ICPC1(31:0)--Output to Idecode indicates the current line PC of thefirst instruction in the 4 issued instructions to pass along with theinstruction to FIROB.

ICPC2(31:0)--Output to Idecode indicates the current line PC of a secondinstruction which cross the 16-byte boundary or branch target in the 4issued instructions to pass along with the instruction to FIROB.

ICPOS0(4:0)--Output to decode unit 0indicates the PC's byte position ofthe next instruction. Bit 4indicates the next instruction is on the nextline.

ICPOS1(4:0)--Output to decode unit 1indicates the PC's byte position ofthe next instruction. Bit 4indicates the next instruction is on the nextline.

ICPOS2(4:0)--Output to decode unit 2indicates the PC's byte position ofthe next instruction. Bit 4indicates the next instruction is on the nextline.

ICPOS3(4:0)--Output to decode unit 3indicates the PC's byte position ofthe next instruction. Bit 4indicates the next instruction is on the nextline.

BTAG1N(3:0)--Output indicates the position of the first target branchinstruction for a new line with respect to the global shift register incase of branch mis-prediction.

BTAG2N(3:0)--Output indicates the position of the second target branchinstruction for a new line with respect to the global shift register incase of branch mis-prediction.

BTAKEN1(1:0)--Output to decode units and ICFPC indicates a predictedtaken branch instruction from PTAKEN, BVAL1. Bit 0 is the last line andbit 1 is new line.

BTAKEN2(1:0)--Output to decode units and ICFPC indicates a predictedtaken branch instruction from PTAKEN, BVAL2. Bit 0 is the last line andbit 1 is new line.

ICERROR--Output, indicates an exception has occurred on an instructionpre-fetched, the type of exception (TLB-miss, page-fault, illegalopcode, external bus error) will also be asserted.

INSPFET--Output to BIU and CMASTER requests instruction fetching fromthe previous incremented address, the pre-fetch buffer in the Icache hasspace for a new line from external memory.

ICAD(31:0)--Output to MMU indicates a new fetch PC request to externalmemory.

RETPRED--Output to Idecode indicates the current prediction of thereturn instruction of the fetched line. The return instruction must bedetected in the current line of instruction or the Icache must bere-fetched from a new line.

IC₋₋ EXT₋₋ RD--Output to CMASTER indicates the next line of instructionshould be fetched from external regardless of aliasing. This is forpre-fetching of instruction which crosses the line boundary.

MMUPFPGFLT--Input from MMU indicates page fault for the currentinstruction address.

TLB₋₋ MISS₋₋ PF--Input from MMU indicates TLB miss for the currentinstruction address.

PF₋₋ IC₋₋ XFER--Output to CMASTER indicates the address for the currentline is written into the cache, the L2 should be updated with thephysical address. This is when the ICPDAT and the valid bit is written.

BIU₋₋ NC--Input from BIU indicates the current line should not becached.

LS2ICNOIC--Input from LSSEC indicates no caching, pre-fetch only.

LS₋₋ CS₋₋ WR--Input from LSSEC indicates the CS is being updated.

L2₋₋ IC₋₋ INV(1:0)--Input from CMASTER to invalidate up to 2 lines inthe Icache.

PF₋₋ IDX(6:0)--Input from CMASTER indicates the array index forinvalidating up to 2 lines in the Icache or for aliasing.

PF₋₋ SNP₋₋ COL(2:0)--Input from CMASTER indicates the way associativefor invalidating up to 2 lines in the Icache or aliasing. This signalmay be redundant with PFREPLCOL(2:0).

BIT20MASK--Input from CMASTER indicates masking of bit 20 for backwardcompatible with 8086. The line should not be cache if outside of thepage.

BSTRUN--Input from TAP indicates to start the BIST.

BSTRD--Input from TAP indicates to read the array and compare to set theresult.

BSTWR--Input from TAP indicates to write the array from input registers.

BSTRST--Input from TAP indicates to reset the counter.

BSTINCR--Input from TAP indicates to increment the counter.

BSTDIN--Input from TAP indicates the test pattern to the inputregisters. The input can be from the TDI pin or normal burn-in patterns.

FLUSHON--Input from TAP indicates flushing register mode, the resultlatch should use BSTDIN instead of the compare input for flushing theresult registers.

UPDOWN--Input from TAP indicates counting up or down.

BSTSHF1--Input from TAP indicates shifting of the master latch ofregisters.

BSTSHF2--Input from TAP indicates shifting of the slave latch ofregisters.

BSTFALSE--Input from TAP indicates to invert the test pattern.

PORTSEL--Input from TAP indicates to select the second dual port.

BSTIDOUT--Output to TAP indicates the result of the data chain from theICSTORE and ICPDAT arrays.

BSTITOUT--Output to TAP indicates the result of the data chain from theICNXTBLK and ICTAGV arrays.

BSTAMSB--Output to TAP indicates maximum count for dual port arrays.

MAXADDR--Output to TAP indicates maximum index counter.

ATPGIN(15:14)--Input from dedicated pins for ATPG.

ATPGOUT(15:14)--Output to dedicated pins for ATPG.

ICSTORE

Processor 500 executes fast X86 instructions directly, no ROPs areneeded. The pre-decode bits with each byte of instruction are 3 bits;start bit, end bit, and functional bit. All the externally fetchedinstructions will be latched into the Icache. This should not be aproblem since the Icache is idle and waits for external instructions.Only single byte prefix of 0x66 and 0x0F is allowed for Processor 500'sfast path, multiple prefixes including 0x67 is allowed for multi-prefix,all other prefixes will take an extra cycle in decoding or go to MROM.With these simple prefixes, the instruction bytes need not be modified.The linear valid bit is used for the whole cache-line of instructions,16-byte. The replacement procedure is done by the CMASTER. Along witheach line of instruction, the CMASTER tells the Icache which way to putin the data and tag. The start and end bits are sufficient to validatethe instruction. If branching to the middle of the line or instructionswrapping to the next line, the start and end bits must be detected foreach instruction or else the instruction must be pre-decoded again. Thepossible cases are branching to the opcode and skipping the prefix(punning of instruction) and part of the wrapping instruction isreplaced in the Icache. The instructions must first pass through thepre-fetch buffers before sending to the ICPRED. The ICPRED has only oneinput from the IB(127:0) for both the pre-fetched or cachedinstructions. The pre-decode information is written into the ICPDAT asthe whole line is decoded. The output IB(127:0) is merged with theprevious 8-byte to form a 24-byte line for the alignment unit to selectand send to 4 decode units.

Since the instruction fetching from external memory will be writtendirectly into the Icache, the pre-fetch buffer should be built into theICSTORE; the input/output path of the array. In this way, the data willbe written into the Icache regardless of the pre-decode information orthe taken branch instruction and the instructions is available to theIcache as soon as they are valid on the bus. The number of pre-fetchbuffers is two, and request will be made to BIU as soon as there isspace in the pre-fetch buffer for another line of instructions. Thepre-fetch buffer consists of a counter and a valid bit for instructionswritten into the cache and a valid bit for instructions sent to thedecode unit. As long as the address pointer is still in the same block,the data will be written to the array. With the pre-fetch buffer in theIcache, a dedicated bus should be used to transfer instructions directlyfrom the pads to the Icache; this is a step to keep Processor 500 fromusing dynamic pre-charged buses.

ICSTORE ORGANIZATION

The ICSTORE on Processor 500 does not store the pre-decode data, asshown in FIG. 9. The ICSTORE consists of 32K bytes of instructionsorganized as 8 sets of 128 rows by 256 columns. The array set in thisdocumentation has its own decoder. The decoder is in the center of theset. Each of the sets consist of 2-byte of instructions. The 8-wayassociative muxing from the 8 TAG-HITs is performed before the data isrouted to the ICALIGN block. With this arrangement, the input/output toeach set is 16-bit buses. The muxing information relating to which byteis going to which decode unit should also be decoded; this topic will bediscussed in detail below in the ICALIGN block section. For optimalperformance the layout of the column should be 64 RAM cells, pre-charge,64 RAM cells, write buffer and senamp. The row decoder should be in themiddle of the array to drive 128 column each way. Basically, thepre-charge and the row decoder should be crossed in the middle of thearray. The self-time column is used to generate internal clock for eachset of the array. Pre-charge is gated by ICLK. The instruction is validby the end of ICLK, the data muxes by the TAG-HIT should be gated byICLK to be valid for the second ICLK. The two-entry pre-fetch buffersare implemented inside the array with data can be written from eitherentry. The output IB bus is taken from either the array or the pre-fetchbuffer.

The BIST for the ICSTORE array uses 128 TAEC cells for reading andwriting the arrays, as discussed further below.

Signal List

ISADD(11:2)-2--Input from ICFPC indicates the address of instruction toaccess the array. Bits 11:5 are for the row decoder, bits 4:2 are forcolumn select. For normal operation, bits 11:4 is used to read/write16-byte only. For special register operation, bits 11:2 is used toread/write 4-byte.

TAGHIT(7:0)-2--Input from ICTAGV indicates which set is selected to readinstructions.

ICSRD-2--Input from ICCNTL to read instruction.

ICSWR-2--Input from ICCNTL to write instructions from pre-fetch buffersinto the array. This signal is asserted at starting of the pre-fetchenable and de-asserted when a pre-fetch buffer entry is valid, theinstruction is written into the array and the valid bit is reset.

GETNEWL-2--Input from ICPRED indicates that pre-decoding has reached theend of the line or encountered a taken branch instruction, a new line ofinstructions is needed in the next cycle. The ICSTORE should send a newline onto IB bus and write into the array.

SRSRD-2--Input from ICCNTL to read instruction from the array in thenext cycle.

SRSWR-2--Input from ICCNTL to write instruction to the array in the nextcycle.

ICSRBB(31:0)-14--I/O from SRB indicates the special register address forthe array or data transferring to/from the SRB.

SETSEL(7:0)-14--Input from ICFPC indicates which set to read, no tagcompare is needed.

TAGCHK-14--Input from ICCNTL to indicates the valid set is from TAGHITor SETSEL.

PBENAB-14--Input from ICCNTL to enable the pre-fetch buffer to latch theINSB(31:0) bus and write into the array.

INSTRDY-13--Input from BIU to indicates the valid external fetchedinstruction is on the INSB(63:0) bus.

INSTFLT-13--Input from BIU to indicates the valid but faulted externalfetched instruction is on the INSB(63:0) bus.

INSB(63:0)-3--Input from external buses for fetched instruction to theIcache.

IB(127:0)-1--Output to ICALIGN after the set select to aligninstructions to decode units.

PBFLT-15--Output to ICCNTL indicates a valid faulted line of instructionfrom external fetch.

PBVAL-15--Output to ICCNTL indicates a valid line of instruction fromexternal fetch.

PBEMPTY-15--Output to ICCNTL indicates the pre-fetch buffer is empty.

PBONE-15--Output to ICCNTL indicates the pre-fetch buffer has oneavailable entry.

PBFULL-15--Output to ICCNTL indicates the pre-fetch buffer is full.

BSTRD--Input from TAP indicates to read the array and compare to set theresult.

BSTWR--Input from TAP indicates to write the array from input registers.

BSTDIN--Input from TAP indicates the test pattern to the inputregisters. The input can be from the TDI pin or normal burn-in patterns.

FLUSHON--Input from TAP indicates flushing register mode, the resultlatch should use BSTDIN instead of the compare input for flushing theresult registers.

BSTSHF1--Input from TAP indicates shifting of the master latch ofregisters.

BSTSHF2--Input from TAP indicates shifting of the slave latch ofregisters.

BSTFALSE--Input from TAP indicates to invert the test pattern.

BSTSTOUT--Output to ICPDAT indicates the result of the data.

ICPDAT

In Processor 500, the pre-decode data is in a separate block than theICSTORE, and is updated with a different timing than that ofinstructions. The ICPDAT is updated as the whole instruction line iscompleted decoding in the ICPRED. To prevent erroneous setting of thepre-decode bits, the valid bit in the ICTAGV is not set until thepre-decode data is written into the array. The pointer to ICSTORE canadvance before the whole line of instruction is completed in pre-decodefor split-line instruction. The pre-decode data includes three bits:start-byte, end-byte, and functional-byte. During pre-decoding, theICPDAT uses 2 sets of latches to keep the pre-decode information foreach line of instructions. The first set is for the current line and thesecond set is used for split-line instructions. As each instruction iscompleted in pre-decoding, the start and end pointers set the latches.The functional bits are kept in the ICPRED and write into the array atthe completion of pre-decoding for the line. If a taken branchinstruction is detected in ICPRED, the control should allow one clockcycle to write the current pre-decode data into the ICPDAT array. Aspecial case of split-line taken branch instruction, the control allows2 clock cycles to write the two pre-decode lines into the array.

The start-byte and the end-byte are scanned to align the instructionsfrom ICSTORE to the decode units. A valid instruction should start witha start byte and end with an end byte before another start byte can bedetected. Any invalid instruction should be detected in the scanninglogic before sending the instruction to early decoding. The invalidinstruction must be pre-decoded again by the ICPRED block. ICPDAT shouldkeep all pre-decode data for the line of the confused instruction fromthe array which will be ORed with the functional bits from pre-decodingfor writing back into the array. The pre-decoding stops on twoconditions: a taken branch is detected or valid start byte (or validinstruction) is detected after the end byte from pre-decoding; thiscondition should be detected in the ICPDAT.

The start bits are decoded into byte-shifting information for up to 4instructions per 8-byte block which will be used by the ICALIGN block.The start-byte pointer for the current fetching instruction is used toreset all the start and end bits before the current pointer, and thetaken branch pointer from the way prediction is used to reset all thestart and end bits after the branch pointer. All 8-way data should belatched in case of way mis-prediction, the corrected way should be readagain from the latches for scanning. An 8-byte or 16-byte block can beread from the array to merge with the previous data to form a 16-byteline for scanning and sending to early decoding. The scanning of thestart bits is sectioned into two 8-byte blocks, the maximum scanned pathis 8-byte long. The scanning outputs control the muxing of up to 7 bytesfor each instruction to early decoding unit. In addition, the scanningof the 8-start-byte generates the look-ahead controls of next 8-byteblocks.

ICPDAT ORGANIZATION

For each 8-byte, the ICPDAT (as shown in FIG. 10) generates 4 sets ofvalid bits based on the start and end bits. The valid bits are muxed tothe ICALIGN the same way as the instruction bytes. The valid bits areused to qualify the functional bits. Example of the valid bits for8-byte instruction:

    ______________________________________                                        Start bits         10100100 01110101                                          End bits           01001000 11101010                                          Valid bits 1       11000000 01000000                                          Valid bits 2       00111000 00100000                                          Valid bits 3       00000111 10011000                                          Valid bits 4       00000000 00000110                                          ______________________________________                                    

In setting up the valid bits for the instructions, ICPDAT needs tovalidate the instruction with proper the start and end bits. Onlyinstructions with valid start and end bits can be in ICALIGN. If theline is hit in the cache, the pre-decode data should be latched into thebuffer for future reference. ICPDAT notifies ICCNTL to pre-decode theinvalid instruction. The functional bits from ICPRED are ORed with thelocal buffer of functional bits for writing back into the array. Duringpre-decoding if a valid start byte is detected after an end byte, ICPDATshould notify ICCNTL to stop pre-decoding, writing the currentpre-decode data into the array, and re-fetching the line for scanning.The scanning logic should use only the pre-decode data from the array.The MROM instruction should also be detected in ICPDAT with a pointer tothe ICALIGN and the MROM. The MROM instruction is read directly from theIB(127:0). MROM is responsible to detect and decode the prefixes andopcode as with ICALIGN. Exemplary hardware is shown in FIG. 11.

The Icache line sets up as two 8-byte sections, the scanned logic andfetching of next line should be independent of each other. Whilescanning, the end-byte should also be detected to indicate that all8-byte can be dispatched in one clock cycle and to fetch a new line.Conditions to fetch the next 8-byte are:

Three start bits or less.

Four start bits with no more than 3 end bits.

Four start bits with 4 end bits, the last end bit is on the last byte.

Hold indication from early decoding.

Hold for more than one MROM instruction in the current 16 bytes.

Since the instruction can have up to 15 bytes which can span more thantwo 8-byte blocks, the ICPDAT can send the instruction to the next stagewith assumed valid bits. The split-line instruction has indication toread the rest of the instruction in the next clock cycle. In the nextcycle if the end bit is not detected within 15 bytes, the instructionmust be sent to ICPRED for pre-decoding. The output from the ICSTORE isIB(127:0), ICPDAT provides the controls for muxing of this bus to theIB1(191:0) which is in the ICALIGN block. The muxing is for any of the8-byte block on IB(127:0) to any of the 8-byte block on IB1(191:0).ICPRED also provides similar controls for the muxes.

If there are more than 4 instructions in the 8-byte block, the ICPDATmuxes the 8-byte block from IB(127:0) to IB1(191:0) and stalls the next8-byte block. In the next clock cycle, the first 4 instructions clearthe pre-decode data and the 8-byte block starts scanning with the fifthinstruction. The same 8-byte block should be muxed from IB(127:0) toIB1(191:0) again. This mechanism keeps the ICALIGN from feeding backinformation on dispatching of the first 4 instructions. With this methodthe scanning of instructions in ICPDAT is independent of the next stageand is more flexible; either 3 or 4 instructions can be scanned.

The scanning logic for each 8-byte segment of instructions is done inparallel with the TAGHITs in the ICTAGV block. More details regardingthe scanning logic may be found in the commonly assigned, co-pendingpatent application entitled "A Parallel and Scalable InstructionScanning Unit", Ser. No. 08/475,400 filed Jun. 7, 1995 by Narayan. Thedisclosure of this patent application is incorporated herein byreference in its entirety. It is possible to implement the fetching,scanning, early decoding, and shifting of instructions to the decodeunits in 4 clock cycles.

MROM INTERFACING

The ICPDAT sends the MROM instruction directly to the MENG. Only oneMROM instruction can be sent to the MENG per clock cycle. The MENGpipelines the MROM instructions and provides latches to keep up to 3MROM instructions including the last MROM on IB(127:0). MROMFULLindicates that the next MROM instruction should be stalled in ICPDATuntil the previous MROM instruction is dispatched to a decode unit.ICPDAT provides the byte pointer for the MENG to read the instructionfrom the IB(127:0) along with the functional and valid bits for decodingthe instruction. ICPDAT should indicate MROM instruction to MENG only ifthe associated 8-bytes can be shifted to the next stage. If theinstruction continues to the next line, MENG can expect to read the restof the instruction on IB(127:0) in the next cycle (similar to sendingsplit-line instruction to ICALIGN). The signal MROMFULL is used to stopICPDAT to keep the data on IB(127:0) for MENG to process prefixes of theinstruction. The micro-instructions rejoin the instruction pipeline inthe muxing stage to the decode units. With early detection of MROMinstruction to MENG, the penalty for MROM instruction is one clockcycle. MENG is responsible to decode prefixes and all fields ofinstructions. The same decoding logic is used in the ICPRED and ICALIGN.

In ICALIGN, as the MROM instruction is detected, it moves to dispatchedposition 0 and waits for micro-instructions from MENG. The followinginstructions fill the pipeline and dispatch as soon as the lastmicro-instruction is sent to decode unit. In dispatch position 0, theMROM instruction should start in the first 8-byte block of theIB2(191:0) bus. The MENG can provide pointers to read the displacementand immediate fields of the MROM instruction directly from theIB2(191:0).

ICPDAT ORGANIZATION

The ICPDAT includes of 32K of 3-bit pre-decode data organized as 8 setsof 64 rows by 192 columns. Each of the sets consists of two 3-bitpre-decode data. The pre-decode data is decoded into byte-shiftinginformation which ise used by the ICALIGN block. The 8-way associativemuxing from the 8 TAGHITs is performed before the byte-shifting data isrouted to the ICALIGN block. In order for the instructions to get to theIdecode in middle of the second ICLK, the decode logic for thebyte-shifting should be less than 7 gates. The byte-shifting logic hasbeen done. Because of this byte-shifting logic, the array for ICPDAT is64 rows instead of 128 rows for the ICSTORE array. For optimalperformance the layout of the column should be 32 RAM cells, pre-charge,32 RAM cells, write buffer and senseamp. The row decoder should be inthe middle of the array to drive 96 column each way. Basically, thepre-charge and the row decoder should be crossed in the middle of thearray. The self-time column is used to generate internal clock for eachset of the array. Pre-charge is gated by ICLK. The byte-shifting datamuxed by the TAGHIT should be gated by ICLK to be valid for the secondICLK. The output of the array should include logic to feedback theprevious pre-decode data for breaking up of the line for second cycleaccess.

The BIST for the ICDAT array uses 48 TAEC cells for reading and writingthe arrays as explained further below.

Signal List

IPADD(11:2)--Input from ICFPC indicates the address of instruction toaccess the array. Bits 11:6 are for the row decoder, bits 5:2 are forcolumn select. For normal operation, bits 11:4 is used to read/write16-byte only. For special register operation, bits 11:2 is used toread/write 4-byte.

TAGHIT(7:0)--Input from ICTAGV indicates which set is selected to readinstructions.

ICSRD--Input from ICCNTL to read instruction.

ICPWR--Input from ICCNTL to write pre-decoded data from ICPRED into thearray. This signal is asserted at starting of the pre-fetch enable andwhen ENDLINE is asserted, the pre-decode data is written into the array.

GETNEWL--Input from ICPRED indicates that pre-decoding has reached theend of the line or encountered a taken branch instruction, a new line ofinstructions is needed in the next cycle. Used to clear the pre-decodelatch.

ENDLINE--Input from ICPRED indicates that pre-decoding has reached theend of the line or encountered a taken branch instruction, thepre-decode data should be written into the array.

SRPRD--Input from ICCNTL to read pre-decode data for special register.

SRPWR--Input from ICCNTL to write pre-decode data for special register.

ICSRBB(23:0)--I/O from SRB indicates the special register address forthe array or data transferring to/from the SRB. 8-byte of pre-decodedata is transferred at one time. The order is Start, Functional, Endbits for each byte of instruction.

SETSEL(7:0)--Input from ICFPC indicates which set to read, no tagcompare is needed. This signal is from SRB or branch prediction.

TAGCHK--Input from ICCNTL to indicates the valid set is from TAGHIT orSETSEL.

NEWWAY--Input from ICCNTL to indicates way mis-prediction to select thecorrect way from the latches using TAGHIT.

ENDINST--Input from ICPRED indicates that pre-decoding is completed forthe current instruction. The pre-decode bits should be set at this time.This signal should also be sent along with ENDLINE for partially decodeinstruction.

PENDIB(4:0)--Input from ICPRED indicates the end byte position of thecurrent instruction. The end bit is set and latched until pre-decodingof the whole line is completed. Bit 4indicates if the end bit is on thenext line.

PFUNCB(15:0)--Input from ICPRED indicates the functional bits of currentline of instructions. ICPDAT uses ENDLINE to OR these bits previouspre-decode data and write into the array. As a new line is read from thearray the pre-decode data is latch in the buffer. The ICPRED must keepall the functional bits for all the pre-decoded instruction in the line.

STARTPTR(3:0)--Input from ICFPC indicates the byte position of thepre-decoded bytes for current instruction. The start byte should be setat this pointer for writing. For reading, this pointer is decoded into16 bits to zero-out all bytes before the pointer for scanning logic.

WAYBPTR(3:0)--Input from ICNXTBLK indicates the byte position of thebranch taken pointer of way prediciton. For reading, this pointer isdecoded into 16 bits to zero-out all bytes after the pointer forscanning logic.

NEXTB(1:0)--Input from ICALIGN indicates that ICALIGN can accept any8-byte block. This signal includes other hold inputs from MROM andIdecode.

NEWLINE(1:0)--Output to ICALIGN and ICFPC indicates the first (bit 0) orthe second (bit 1) 8-byte is the starting of a new line. The currentline can start at the second 8-byte. The PC and offset of instructionssending to the decode units should be adjusted accordingly.

VALSTART--Output to ICCNTL indicates a valid start-byte after the endpointer from ICPRED for the current lines of instructions. Thepre-decoding stops and instructions is fetched from the array.

IC0POS1(3:0)--Output to ICALIGN indicates the PC's byte position of thenext instruction using the end bit for instruction 0 of the first8-byte.

IC1POS1(3:0)--Output to ICALIGN indicates the PC's byte position of thenext instruction using the end bit for instruction 1 of the first8-byte.

IC2POS1(3:0)--Output to ICALIGN indicates the PC's byte position of thenext instruction using the end bit for instruction 2 of the first8-byte.

IC3POS1(3:0)--Output to ICALIGN indicates the PC's byte position of thenext instruction using the end bit for instruction 3 of the first8-byte.

IC0POS2(3:0)--Output to ICALIGN indicates the PC's byte position of thenext instruction using the end bit for instruction 0 of the second8-byte.

IC1POS2(3:0)--Output to ICALIGN indicates the PC's byte position of thenext instruction using the end bit for instruction 1 of the second8-byte.

IC2POS2(3:0)--Output to ICALIGN indicates the PC's byte position of thenext instruction using the end bit for instruction 2 of the second8-byte.

IC3POS2(3:0)--Output to ICALIGN indicates the PC's byte position of thenext instruction using the end bit for instruction 3 of the second8-byte.

IC0VAL1(7:0)--Output to ICALIGN indicates valid bytes for instruction 0of the first 8-byte. The valid bytes are used for muxing of the bytes toearly decoding. The valid bytes can continue to next 8-byte blocks untilthe end bit is detected.

IC1VAL1(7:0)--Output to ICALIGN indicates valid bytes for instruction 1of the first 8-byte. The valid bytes are used for muxing of the bytes toearly decoding. The valid bytes can continue to next 8-byte blocks untilthe end bit is detected.

IC2VAL1(7:0)--Output to ICALIGN indicates valid bytes for instruction 2of the first 8-byte. The valid bytes are used for muxing of the bytes toearly decoding. The valid bytes can continue to next 8-byte blocks untilthe end bit is detected.

IC3VAL1(7:0)--Output to ICALIGN indicates valid bytes for instruction 3of the first 8-byte. The valid bytes are used for muxing of the bytes toearly decoding. The valid bytes can continue to next 8-byte blocks untilthe end bit is detected.

IC0VAL2(7:0)--Output to ICALIGN indicates valid bytes for instruction 0of the second 8-byte. The valid bytes are used for muxing of the bytesto early decoding. The valid bytes can continue to next 8-byte blocksuntil the end bit is detected.

IC1VAL2(7:0)--Output to ICALIGN indicates valid bytes for instruction 1of the second 8-byte. The valid bytes are used for muxing of the bytesto early decoding. The valid bytes can continue to next 8-byte blocksuntil the end bit is detected.

IC2VAL2(7:0)--Output to ICALIGN indicates valid bytes for instruction 2of the second 8-byte. The valid bytes are used for muxing of the bytesto early decoding. The valid bytes can continue to next 8-byte blocksuntil the end bit is detected.

IC3VAL2(7:0)--Output to ICALIGN indicates valid bytes for instruction 3of the second 8-byte. The valid bytes are used for muxing of the bytesto early decoding. The valid bytes can continue to next 8-byte blocksuntil the end bit is detected.

ICENDB1(3:.0)--Output to ICALIGN indicates if end-bits for 4instructions are detected in the first 8-byte block. If the end-bit isnot set, the instruction continue to the next 8-byte blocks.

ICENDB2(3:0)--Output to ICALIGN indicates if end-bits for 4 instructionsare detected in the second 8-byte block. If the end-bit is not set, theinstruction continue to the next 8-byte blocks.

ICFUNC(15:0)--Output to ICALIGN and MENG indicates functional-bits, bits7:0 are for the first 8-byte block and bits 15:8 are for the second8-byte block.

INVPTR(3:0)--Output to ICFPC indicates the start pointer of the invalidinstruction which needs to be pre-decoded.

INVINST--Output to ICFPC and ICCNTL indicates the invalid instructionwhich needs to be pre-decoded. No valid byte should be sent to theICALIGN.

MROMPTR(3:0)--Output to ICFPC and MROM indicates the start pointer ofthe MROM instruction

ICMROM(1:0)--Output to MENG, bit 0indicates the current instruction isMROM, bit 1indicates the MROM instruction continue to the next line(end-bit is not detected in the current line).

MROMFUL--Input from MENG indicates MENG cannot accept any moreinstruction.

IC8BSEL(5:0)--Output to ICALIGN controls the muxes from IB(127:0) toIB1(191:0). Bit 5:3 is for the upper 8-byte IB(127:64) and bit 2:0 isfor the lower 8-byte IB(63:0). 001 is for muxing to IB1(63:0), the first8-byte, 010 is for muxing to the second 8-byte, and 100 is for muxing tothe third 8-byte.

INVAL2--Output to ICALIGN to invalidate the second 8-byte block whichhas been muxed to IB1(191:0) because the first 8-byte has more than 4instructions or the taken branch invalidates the second 8-byte block.

BSTRD--Input from TAP indicates to read the array and compare to set theresult.

BSTWR--Input from TAP indicates to write the array from input registers.

BSTSTOUT--Input from ICSTORE connects the BIST chain.

FLUSHON--Input from TAP indicates flushing register mode, the resultlatch should use BSTDIN instead of the compare input for flushing theresult registers.

BSTSHF1--Input from TAP indicates shifting of the master latch ofregisters.

BSTSHF2--Input from TAP indicates shifting of the slave latch ofregisters.

BSTFALSE--Input from TAP indicates to invert the test pattern.

BSTIDOUT--Output to TAP indicates the result of the data chain from theICSTORE and ICPDAT arrays.

ICTAGV

Processor 500 executes the fast X86 instructions directly and theinstructions are written into the Icache regardless of the pre-decodeinformation. The linear valid bit is used for the whole line ofinstructions, assuming that the BIU always fetches 16-bytes of data. TheCMASTER indicates which way to put in the pre-fetch data and tag.Writing of the tag is done at the same time as writing the data into theICSTORE, and writing of the linear valid bit is done at the same time aswriting the pre-decode bits into the ICPDAT. The start and end bits aresufficient to validate the instruction. If branching to the middle ofthe line or instructions wrapping to the next line, the start and endbytes must be detected for each instruction or else the instruction mustbe pre-decoded again. The possible cases for invalid instructions are(1) branching to the opcode and skipping the prefix, (2) part of thewrapping instruction are replaced in the Icache, (3) part of the linewas not pre-decoded because of branch instruction and branch target.Whenever the MMU is re-mapped or the CMASTER execute a certaininstruction, all the LV bits can be cleared. The next access to theIcache would results into LV miss, the CMASTER may send new mappinginformation to the Icache regarding the way associative, the SU, and thenew tag. In this case the Icache needs to write the ICTAGV with newinformation (including setting the LV bit) and read the other arrays.

From the branch prediction, tag address must be read from the ICTAGV inthe next cycle to merge with the successor index for the predictedtarget address. The predicted target address must be sent to the decodeunits and to the functional units for comparison.

ICTAGV ORGANIZATION

The ICTAGV includes 2048 lines of:

20-bit Tag address.

2 Status bits (SU, LV).

The status bits need to be dual-port to read and write in the same clockcycle. The ICTAGV is organized as two sets of 64 rows by 224 columns andtwo sets of 64 rows by 128 columns. Each of the first two sets consistsof 7-bit tag address, and each of the last two sets consists of 3-bittag address and the SU or LV bit. The two status bits are dual port RAMcells. The SU uses the delayed PC to write, and the LV bit has thesnooping index from CMASTER. The ICTAGV uses 64 rows for dual-port RAMand quick reading of tag address. For optimal performance the layout ofthe column should be 32 RAM cells, pre-charge, 32 RAM cells, writebuffer and senamp. The row decoder should be in the middle of the arrayto drive 112 or 96 columns each way. Basically, the pre-charge and therow decoder should be crossed in the middle of the array. The rowdecoder for the dual port RAM should be build at one end of the array.The self-time column is used to generate internal clock for each set ofthe array. Pre-charge is gated by ICLK. The status bits muxed by theTAGHIT should be gated by ICLK to be valid for the second ICLK. Theabove layout is to ensure the minimum routing for the TAGHIT, and isshown in FIG. 12.

The BIST for the ICTAGV array uses 22 TAEC cells for reading and writingthe arrays, as explained further below.

Signal List

ITADD(11:4)--Input from ICFPC indicates the address of instruction toaccess the array. Bits 11:5 are for the row decoder, bit 4 is for columnselect. Writing into the SU array is a delay of the index to next ICLK.

IVADD(11:4)--Input from ICFPC indicates the address of instruction towrite the LV bit. Bits 11:5 are for the row decoder, bit 4 is for columnselect.

ICTAG(31:12)--Input from ICFPC indicates the address of instruction tocompare with the tag arrays.

ICTVRD--Input from ICCNTL to read tag array.

ICTWR--Input from ICCNTL to write new tag.

ICCLRA--Input from ICCNTL to clear all valid bits.

ICSUWR--Input from ICCNTL to write the SU bit.

ICLVWR--Input from ICCNTL to write the LV bit.

SRTVRD--Input from ICCNTL to read tag for special register.

SRTVWR--Input from ICCNTL to write tag for special register.

ICSRBB(31:10)--I/O from SRB indicates the special register address forthe array or data transferring to/from the SRB. Bits 31:12 are theaddress, bit 11 is the supervisor/user, and bit 10 is the valid bit.

SETSEL(7:0)--Input from ICFPC indicates which set to read, no tagcompare is needed.

TAGCHK--Input from ICCNTL to indicates the valid set is from TAGHIT orSETSEL.

TAGHIT(7:0)--Output indicates which set is selected to readinstructions.

VALBIT(7:0)--Output indicates the valid bits of 8 sets, uses forway-prediction.

TVSU--Output indicates the supervisor/user mode of current block.

IBTARGET(31:12)--Output to decode unit indicates the predicted takenbranch target for the line on instruction in the previous cycle. Thepredicted taken branch from ICNXTBLK provides only the index, the upperbits are read from ICTAGV in the next cycle.

BSTRD--Input from TAP indicates to read the array and compare to set theresult.

BSTWR--Input from TAP indicates to write the array from input registers.

BSTNBOUT--Input from ICNXTBLK connects the BIST chain.

FLUSHON--Input from TAP indicates flushing register mode, the resultlatch should use BSTDIN instead of the compare input for flushing theresult registers.

BSTSHF1--Input from TAP indicates shifting of the master latch ofregisters.

BSTSHF2--Input from TAP indicates shifting of the slave latch ofregisters.

BSTFALSE--Input from TAP indicates to invert the test pattern.

PORTSEL--Input from TAP indicates to select the second dual port.

BSTITOUT--Output to TAP indicates the result of the data chain from theICNXTBLK and ICTAGV arrays.

ICNXTBLK

This block contains the branch prediction information for the Icache. Animportant feature for performance of superscalar/superpipelinedmicroprocessors is branch prediction. As the number of pipeline stagesand the number of functional units increase the cost of mis-predictionis high. The tag comparison and selection of the 8-way associative inICTAGV takes more than one clock cycle to resolve. Processor 500implements another prediction technique to pick one out of the 8 waysand to use the predicted way for fetching of next line to remove thebubble in the pipeline. Three bits are needed for this way prediction.Another two bits are needed to select one of the two branch targets forfetching the next predicted line. Without the two bits to select thebranch targets, comparison of the start-byte position with the positionsof the branch targets will cause the way-prediction to be more than oneclock cycle. This speculatively selected line of instructions can becorrected in the next cycle from the proper TAGHIT and the branchprediction. The following rules are used to set up the ICNXTBLK:

During pre-decode, if there is an unconditional branch instruction, thefetching is stalled until the new target PC can be calculated. If thebranch is direct, the target PC can be calculated in the next cycle inthe ICFPC.

The backward taken conditional branch which the new target PC can becalculated in the ICPFC should also predict taken.

If the branch is indirect, the target PC is from the functional unit.The speculative way-prediction is updated with the successor index inthe cycle following the target address access. If there is nopre-decoded taken branch, the speculative way-prediction is updatedafter fetching of the next sequential block. The index after accessingthe array should be kept for 3 clock cycles before the way-prediction isknown for updating.

From mis-prediction, the new target PC will be fetched and the selectedset and successor index will be kept in a branch holding registers whichwill be discussed in next section.

For better branch prediction hit ratio, the ICNXTBLK will increase to 2branch targets for both taken and non-taken branches, and pre-decode forthe unconditional branches and taken branches. The pre-decoding alsocalculate the branch target address for the branch instructions ifavailable. If there is a hit in the ICNXTBLK, the new target will beused for the new fetch PC. The taken branch will have higher priority tooccupy the two targets in ICNXTBLK.

Processor 500 will implement two different branch predictors to maximizethe performance. The next few sections discuss the Bimodal and Globalpredictors and the implementation of the combined branch predictor onProcessor 500.

Bimodal Branch Prediction

This branch prediction uses a saturated counter for prediction. A 2-bitcounter is used for taken/non-taken prediction. The most significant bitis used for taken/non-taken prediction. Since branch instructions aremore likely to be taken than non-taken, the counter should initialize to10 if the branch target address can be calculated. As the branch istaken, the counter increases by 1 and is saturated at 11. As the branchis not taken, the counter decreases by 1 and is saturated at 00. Thebimodal branch prediction is better than a single bit prediction as thebranch correct prediction is 88.09% instead of 82.29% for two targetsprediction. The bimodal branch prediction behaves well for mostly takenor non-taken branch instruction and is worst as the conditional branchesflip-flop between taken and non-taken. The cost is one extra bit perbranch target in the ICNXTBLK, and extra logic for the counter.

Global Branch Prediction

This branch prediction is an independent branch predictor, not a part ofthe Icache. There are many different types of global branch prediction;Processor 500 uses the global branch prediction which has the highestbranch correct prediction ratio. The prediction entries are indexed byan exclusive OR of the PC and the branch shift register, which isreferred to as global sharing branch prediction. This global branchprediction has the branch correct prediction at 89.24%; the predictionimproves as more branch history bits are used in the prediction. Asingle shift register records the direction taken/non-taken by the mostrecent n conditional branches. Since the branch history is global to allbranches, global branch prediction takes advantage of two types ofpatterns, the direction taken by the current branch may depend stronglyon the other recent branches, and the duplicating the behavior of localbranch prediction (patterns of branches in loops). For Processor 500,since the highest priority of the two branch targets is taken branch,the global shift register includes both the conditional andunconditional branches. In a few cases, the non-taken conditionalbranches may not include in the global shift register. To match thenumber of entries in the Icache, the global branch prediction needs tohave 2048 entries with 2 targets per entry. It is organized with 256rows of 8-way associative. Eight bits are used to index the branchprediction table. The PC uses bit 11:4 for indexing the branchprediction table.

Processor 500's Combined Branch Prediction

Combining the Bimodal and Global branch predictions should give a bettercorrect prediction ratio. A predictor counter is used to select whichbranch predictor is better for each individual branch instruction. Thistechnique should give a higher correct prediction ratio than the abovetwo prediction techniques; the prediction ratio can be shown to be89.93%. The same saturated counter is used for the predictor counter: ifthe bimodal predictor is correct and the global predictor is incorrectthen increase the counter until saturated; if the global predictor iscorrect and the bimodal predictor is incorrect then decrease the counteruntil saturated; else no change in the predictor counter. The mostsignificant bit of the predictor counter is used for selecting thebranch predictor. ICNXTBLK is implemented with the bimodal counter andthe predictor counter. ICNXTBLK has 2 targets per instruction line,where each target consists of the following:

12 bits-successor index, need 11:4 for global table index, 11:0 foricache.

3 bits-for 8-way associative.

4 bits-byte position of the branch instruction within the current line.

2 bits-bimodal counter

2 bits-predictor counter

1 bit - valid target

The table for global branch predictor also has 2 targets per entry,where each entry consists of a 2-bit global counter. The bimodal andpredictor counters must be able to update the count on every cycle. Theleast significant bit of the counters is dual ported. If the count is10, predicted taken, the new count is 11. If the count is 01, predictednon-taken, the new count is 00. Only on branch mis-prediction does themost significant bit of the counters change state.

The successor index, way, and byte position are shared between thebimodal and global counters. The predictor counter selects between thebimodal and global counters. The combination of the counters is topredict branch instructions as taken/non-taken. In most cases, theshared target is sufficient for the two branch predictors. The branchmay have two different targets depended on two different patterns ofbranch instructions. In this case, the global predictor may desire tohave its own target. It would be very expensive in layout area to have 4targets per 16 bytes of instruction. A diagram of ICNXTBLK is shown asFIG. 13.

Implementation of Global Branch Tag for Recovery

As discussed above, the global branch predictor needs a table with 256rows, 8-way associative, and 2 targets per line. A shift register of 8bits is needed for indexing. The global branch predictor shift registermust be able to back track to the previous conditional branch which ismis-predicted. As each conditional branch is predicted, the direction ofthe branch is shifted into the shift register from right to left, 1 fortaken and 0 for non-taken. The shift register needs a total of 24 bitsand each of the bit beyond the 8 indexing bits needs to have a tag. Thetag has 4 bits to indicate the conditional branch position in the shiftregister. FIROB has 6 lines of 4 instructions and the pipeline fromfetch to dispatch can hold another 3 lines of 4 instructions for a totalof 36 instructions. The average length of X86 instruction is 3 bytes,thus (36 instruction X 3 bytes) 108 bytes are equivalent to 6.75 linesfrom the Icache. Each Icache line of instructions can have up to 2branch targets, which is 14 branch instructions in the pipeline. Theextra 16 bits in the shift register is sufficient to handle all branchesin the pipeline. The shift register needs to keep track of all branchinstructions. All the branch tags and the taken/non-taken bits areshifted in the shift register. The shift register tag (branch tag) isrouted with the branch instruction to the functional units and FIROB. Asthe branch instruction is mis-predicted the branch tag is used toretreat the old 8 bits in the shift register to restore the shiftregister with new direction for the mis-predicted branch instruction.

Even though it is unlikely that the number of branch instructions in thepipeline exceeds 16, a mechanism is needed to detect the overflow of theglobal shift register. An extra valid bit is set for each branch in theglobal shift register. As the branch instruction is retired, the branchtag is returned to the global shift register to clear all the tail validbits from the branch tag. As the global shift register is shifted, ifthe valid bit at the tail of the shift register is set, then thepipeline must stall for the branch instruction to retire from the FIROB.

In the global shift register, beside the taken/non-taken bits, thebranch predictor count, the bimodal count, the global count, the validbit, and the branch target number should be kept in the same globalshift register which will be restored for updating of the counters andthe branch target number in case of branch mis-prediction. The countersare 6 bits, the valid is 1 bit, the branch target number is 1 bit, thetaken/non-taken is 1 bit, and the branch tag is 4 bits; the total bitsin the shift register is 13. The branch target number is to update oneof the two branch targets in the ICNXTBLK. Since the ICNXTBLK can keeponly two branch targets, some of the branch instructions may not bedetected and may not get a branch tag from the global shift register. Ifthe mis-prediction was not in the branch prediction, a new entry shouldbe setup. The counters will be restored and incremented/decrementedbefore sending to the holding register for updating the mis-predictedblock. The return stack in the ICFPC should also restore frommis-prediction.

The global shift register is set and shifted in the second ICLK as thebranch prediction is determined. In later pipeline stages, theunconditional jump, call, and return instructions can be detected andchange the direction of the fetched instruction. In this case, theglobal shift register must be restored. A new branch tag is setup forthe unconditional jump, call, and return instructions. The only branchinstructions without branch tags are the conditional branch instructionwhich are not recorded in the ICNXTBLK. The ICFPC must keep track of thebranch instructions in the first 4 pipeline stages for recovery of theglobal shift register.

All the branches must be detected in pre-decoding and kept in theICNXTBLK unless there are more than two branches. If the branch ismis-predicted and not detected in the prediction then there are morethan two branches in the same line with the mis-predicted branch. OnProcessor 500, there are two branch targets, if the branchmis-prediction matches one of the target, then the mis-prediction willupdate the same target. If the branch mis-prediction was not in theICNXTBLK, then one of the two branch targets must be selected to bereplaced. Information is needed with each of the line PC, the associatedbranch information from dispatching is:

Taken/non-taken (1/0) for the two branch targets of the current line PC.

Current way selection of the line PC.

Taken/non-taken prediction with each branch instruction.

Global shift register branch tag with each branch instruction.

Valid branch tag indication with each instruction.

Non-taken is assumed for the invalid branch targets in the ICNXTBLK.Branch target bits and way select are sent with the PC to the FIROB. TheFIROB returns the two branch target bits upon mis-prediction forreplacement algorithm of the branch targets in the ICNXTBLK. The firstpriority for replacing the branch targets is non-taken. If both bits areset for the line of the mis-predicted branch, then random replacementbased on the least significant bit of the address is used. For branchmis-prediction, the following information are returned from the FIROB:

Current branch way select.

Valid branch tag indication.

Branch tag.

Current branch index.

Taken/non-taken branch target bits of the line PC.

New branch target address.

A diagram of the global branch predictor is shown as FIG. 14.

Branch-Holding Register for Updating Branch Targets

Some processors use the successor index instead of the full address tolimit the size of the array, the selected way for the successor index isknown after the branch target is fetched from the Icache. Updating theICNXTBLK with successor index and selected way required dual-port RAM.With two targets per line of instructions, the current dual-port RAMcells implementation of the ICNXTBLK would make the ICNXTBLK arraylarger than the ICSTORE array. There are two cases of updating thebranch targets: during pre-decoding of taken branch instruction andbranch mis-prediction. On Processor 500, an extra branch holdingregister for branch mis-prediction and pre-decoding branch instructionwill be implemented. The branch holding register should always becompared to the PC address and forward the data instead of reading fromthe ICNXTBLK. When the next branch mis-prediction occurs, the FIROBsends the logical address PC. The successor index, the way select, andbranch predictor counts are latched inside the ICNXTBLK to write intothe array at a later convenient time. Since the way prediction array isdual-ported, it should be updated immediately instead of keeping in thebranch holding register. The branch holding register will update thelast mis-predicted branch target to the ICNXTBLK as the ICFPC takes onecycle to calculate the new linear address from the logical address.Another condition to write the branch holding register into the ICNXTBLKarray is when external fetch is started. With the branch holdingregister, the ICNXTBLK array can be single-ported which is less thanhalf the size of the dual-ported array. The branch holding registerincludes the branch address which is used for comparison and forwardingof data, the successor index, the update branch predictor count afterreading of the new target line. The branch address register resides inthe ICFPC for comparison to the current fetch PC. If the mis-predictionis a RETURN instruction, only the target selection of the way predictionshould be updated.

The branch predictor counters are kept in the global shift register. Ifthe mis-prediction returns a valid branch tag, then the countersincrease or decrease before latching in the branch holding register. Ifthe mis-prediction does not have a valid branch tag, then the bimodalcounter is used with 10. The global shift register should restore theproper branch pattern and the return stack should remove the cancelledCALL/RETURN instructions.

WAY PREDICTION

Since the evaluation of the branch prediction takes two cycles whichwould create a bubble in the pipeline, the way-prediction is implementedfor faster prediction. The way prediction predicts one of the 8-wayassociative will be hit, and uses the ICNXTBLK for the next fetch PC.The way-prediction is validated in the next cycle with the TAGHIT andthe actual branch prediction. If the way is mis-predicted, a correct waywill be selected for the 8-way latches in the ICPDAT and a new line willbe fetched creating a bubble in the pipeline. Since the tag compare isnot completed until the next clock cycle, the scanning logic should bedone in parallel with the tag compare for optimal timing of fetchinginstruction. The scanning logic is implemented on all 8 ways pre-decodedata and the outputs are selected by TAGHIT. With the way prediction, asingle pre-decode data can be selected for scanning logic thus reducingthe logic to one-eighth. The ICPDAT must implement a set of latches tokeep the output data; in case of way mis-prediction, the correct datawill be used for scanning logic in the next cycle. The branch predictionin the ICNXTBLK should be resolved in parallel with TAGHIT for thecorrect next line fetch in the third cycle. The instructions from theICSTORE are not needed for aligning until the next clock cycle whichwill be selected by TAGHIT. The way select from branch prediction shouldbe used to select the way instead of the way prediction. From branchprediction, there is no tag comparator for the upper address bits.

The way-predicting has 3 bits to mux the successor index and branchprediction from ICNXTBLK for accessing the Icache in the next cycle.Depending the current fetch PC's position, the way prediction can useone of the two branch targets or none for sequential. In order to accessthe next PC within a cycle, a target selection of 2 bits is needed. Thetarget selection can also include the return stack option. The programfor the target selection is:

00--sequential,

01--first branch target,

10--second branch target,

11--return stack.

The way prediction should provide the byte position of the returninstruction so that the ICPDAT can invalidate all followed instructions.For speed, another 4 bits are needed to indicate the byte positionpointer for the instruction after the taken branch instruction forICPDAT. The ICPDAT can decode and invalidate the pre-decode datadirectly from this 4-bit branch pointer. If the 4-bit branch pointer isall zeros, no invalidation of the pre-decode data is needed.

The way prediction includes dual ported RAM cells because theinformation must be updated while reading data, the initial value forthe way prediction should be the same as the current PC's set. The wayprediction is not known for updating until the next clock cycles oruntil the early decoding of the return instruction. For convenience, aholding way register is used for updating of the way prediction in thefifth ICLK.

    ______________________________________                                        Clock                                                                              Latch 1 Latch 2 Latch 3                                                                             Latch 4                                                                              Latch 5                                                                             PC0                                   ______________________________________                                        1    PC0     --      --    --     --    read                                  2    PC1     PC0     --    --     --    branch pred                           3    PC2     PC1     PC0   --     --    return inst.                          4    PC3     PC2     PC1   PC0    --    decoding                              5    PC4     PC3     PC2   PC1    PC0   update way                                                                    pred                                  1    PC0     --      --    --     --    read                                  2    PC1     PC0     --    --     --    way mis-pred                          3    PC2     --      PC0   --     --    return inst.                          4    PC3     PC2     --    PC0    --    decoding                              5    PC4     PC3     PC2   --     PC0   update way                                                                    pred                                  ______________________________________                                    

The way mis-prediction can be at two levels, the first level is thewrong predicted way and the second level is the wrong selected target.Way mis-prediction is from comparing the predicted way to the tagcompares, a mis-prediction would have re-fetched the correct instructionline and a new fetch address as shown in case 1 of FIG. 15. If the wayprediction is correct, the next level is the target selectionmis-prediction, the recovery mechanism is shown in case 2 of FIG. 15.The return stack prediction is not known until ICLK4 to correct thefetch done in ICLK2, other target predictions are compared to the branchprediction in ICLK2 and re-fetch the next address in ICLK3 as shown incase 3 of FIG. 15.

ICNXTBLK ORGANIZATION

The ICNXTBLK consists of 2048 lines of 2 branch targets, each targetconsist of 24 bits:

12 bits--successor index, need 11:4 for global table index, 11:0 foricache.

3 bits--for 8-way associative.

4 bits--byte position.

2 bits--bimodal counter.

2 bits--predictor counter.

1 bit--valid target.

The least significant bits of the bimodal and predictor counters aredual ported for writing back the increment count after reading. Forspeculative prediction, the most significant bit of the counter shouldnot change. The new value of the bimodal and predictor counters arespeculative; if there is a branch mis-prediction, the corrected counterswill be written from the branch holding register.

The ICNXTBLK also includes 2048 lines for way prediction which are dualported:

3 Way-prediction bits.

2 Target-selection bits.

4 Taken branch byte-position bits.

The global branch prediction includes 2048 lines of 2 global counterswhich is organized as 256 rows for 8-way associative. The leastsignificant bit of the global counter is dual ported for reading andwriting the speculative prediction in the next cycle.

The ICNXTBLK is organized as 4 sets of 64 rows by 256 columns, 2 sets of64 rows by 196 columns including some dual-ported columns, 1 set of 64rows by 128 dual-ported column, 1 set of 64 rows by 96 dual-portedcolumns, 1 set of 64 rows by 64 dual-ported columns, and 1 set of globalcounter array. Each of the first two sets consist of 2×4 bits ofsuccessor index. The next two sets consists of 2×4 bits of successorindex and 2×4 bits of the byte position. The next two sets consists of2×2 bits bimodal counter, 2×2 bits predictor counter, and 2×3 bits 8-wayassociative, the least significant bits of the counters are dual-port.The next set consists of 4 bits of way branch byte pointer. The last twosets consist of the 3 bits way-prediction and 2 bits target selectionwhich are dual-ported RAM cells. The least significant bits of thecounters are dual ported to be updated on every cycle. To minimizerouting and implementation of the branch holding register, the sameassociated bits of the two branch targets should be laid out in two setsopposite each other. The branch successor index is selected by the wayand target prediction to access the ICACHE in next clock cycle. Becauseof this speed-path in way prediction for reading the Icache in the nextcycle, the array for ICNXTBLK is 64 rows instead of 128 rows as for theICSTORE array. For optimal performance the layout of the column shouldbe 32 RAM cells, pre-charge, 32 RAM cells, write buffer and senamp. Therow decoder should be in the middle of the array to drive 96 or 112column each way. Basically, the pre-charge and the row decoder should becrossed in the middle of the array. The self-time column is used togenerate internal clock for each set of the array. Pre-charge is gatedby ICLK. The ICNXTBLK has two different outputs; the first output in thefirst cycle is based on the way-prediction, and the second output in thesecond cycle is based on TAGHIT. If the two outputs do not select thesame set or are not both non-taken, the reading of instruction in thesecond cycle will be invalidated, creating a bubble in the pipeline. Thesecond output should be gated with TAGHIT and ICLK to be valid in thesecond cycle.

The branch holding register may be built into the ICNXTBLK array. Thismeans that the bits of the two targets must be alternated in the arrayto access the branch holding register. The array may be skewed toaccommodate the bits into a single array. The global branch counter isalso implemented as the array in ICNXTBLK. The shift register and thebranch tag for mis-prediction recovery are also implemented next to thearray.

The BIST uses 55 TAEC cells for reading and writing the arrays. See thesection on BIST and ATPG. A diagram of ICNXTBLK layout is shown as FIG.16.

Signal List

INADD(11:3)--Input from ICFPC indicates the address of instruction toaccess the array. Bits 11:5 are for the row decoder, bit 4 is for columnselect, bit 3 is for special register read/write to each target. Thisindex should be delayed by 3 cycles to write data for the way predictionbits. This index should be latched into the branch holding register.

ICNVRD--Input from ICCNTL to read branch prediction array.

ICNWR--Input from ICCNTL to write branch prediction. This signal isasserted during pre-decoding, the ENDLINE and not PDTAKEN cause thebranch holding register to be written into the array.

ICBCWR--Input from ICCNTL to write bimodal counter bits. The index is 2cycle delay from INADD.

ICPSWR--Input from ICCNTL to write predictor counter bits. The index is2 cycle delay from INADD.

ICGCWR--Input from ICCNTL to write global counter bits. The index is 2cycle delay from INADD.

ICWWR--Input from ICCNTL to write way prediction bits. The index is 3cycle delay from INADD.

INVBHREG--Input from FIROB to invalidate the branch holding register.The branch mis-prediction is speculative, an early branch can bemis-predicted at a later time.

WRBHREG--Input from ICCNTL to write the branch holding register withinformation from the global shift register or from ICPRED.

WRBHREG1--Input from ICCNTL to write the branch holding register withthe way and successor index from accessing the array with informationfrom the global shift register or from ICPRED. This signal is 1 cyclelater than WRBHREG.

SRNRD--Input from ICCNTL to read branch prediction for special register.

SRNWR--Input from ICCNTL to write branch prediction for specialregister.

ICSRBB(28:0)--I/O from SRB indicates the special register address forthe array or data transferring to/from the SRB. Bits 23:0 are the branchtarget, bit 28:26 is the way-selected prediction , and bit 26:24 isway-target prediction. The array requires bit 3 of the address to readthe whole line.

SETSEL(7:0)--Input from ICFPC indicates which set to read, no tagcompare is needed.

TAGCHK--Input from ICCNTL to indicates the valid set is from TAGHIT orSETSEL.

TAGHIT(7:0)--Input from ICTAGV indicates which set is selected to readbranch array.

GETNEWL--Input from ICPRED indicates that pre-decoding has reached theend of the line or encountered a taken branch instruction, a new line ofinstructions is needed in the next cycle. Use to start with the firsttarget and increase to second target as a branch instruction isdetected.

ENDLINE--Input from ICPRED indicates that pre-decoding has reached theend of the line or encountered a taken branch instruction, thepre-decode data should be written into the array.

ENDINST--Input from ICPRED indicates that pre-decoding is completed forthe current instruction. The pre-decode bits should be set at this time.This signal should also be sent along with ENDLINE for partially decodeinstruction. The byte position of the branch instruction is fromSTARTPTR.

PDJMP--Input from ICPRED indicates the current instruction is a branchinstruction. The branch holding register should be setup untilpre-decoding of the whole line is completed. If there is a taken branchinstruction, the new branch holding register must wait for BWAY and notupdate the ICNXTBLK until the next opportunity. The ICNXTBLK should sendBTAG1 to the decode unit.

PDTAKEN--Input from ICPRED indicates the current instruction is takenbranch (the taken branch target can be calculated). The predictorcounter in the branch holding register should be setup untilpre-decoding of the whole line is completed. The successor index and wayassociative will be sent to the ICPRED by ICFPC. The ICNXTBLK shouldsend BTAKEN1(0) to decode unit.

GSRSHIFT(3:0)--Input from ICFPC indicates restoring the global shiftregister from mis-prediction of the RETURN instruction in decode.

MINDEX(14:0)--Input from ICFPC indicates new successor index from branchmis-prediction or new pre-decode taken branch instruction to latch intobranch holding register.

MBTAG(3:0)--Input from ICFPC indicates the branch tag from branchmis-prediction to restore the global shift register, counters, byteposition, and target number for updating the ICNXTBLK. The target numberindicates which branch target to update. The byte position should notchange for branch mis-prediction.

BWAY(2:0)--Input from ICFPC indicates new way associative from takenbranch mis-prediction or new pre-decode taken branch instruction tolatch into branch holding register and for way prediction. This signalis one cycle later than PDINDEX, MINDEX, or MBTAG. BWAY is accessing thenext line. BTAR(1:0)--Output to ICNXTBLK indicates the target of thecurrent line for way prediction. This signal is three cycle after thecurrent line access.

WAYBPTR(3:0)--Output to ICPDAT indicates the byte position of the branchtaken pointer of way prediciton. For reading, this pointer is decodedinto 16 bits to zero-out all bytes after the pointer for scanning logic.

All the branch prediction signals to the decode units need to mux in thepre-decode branch information and STARTPTR as the first branch target.The branch target address is from the ICFPC as ICPC1 or ICPC2.

BPMSB(1:0)--Internal, indicates taken branch from the msb of the bimodalcount of the two targets.

GPMSB(1:0)--Internal, indicates branch taken from the msb of the globalcount of the two targets.

PSMSB(1:0)--Internal, from the msb of the predictor count of the twotargets, 1 indicates using the bimodal predictor, 0 indicates using theglobal predictor.

PTAKEN(1:0)--Internal, indicates if the branch targets are predictedtaken. This signal is the multiplex of the above three signals.

STARTPTR(3:0)--Input from ICFPC indicates the start byte position of theinstruction, the position of the branch target must be greater than thestart byte. This byte pointer is latched into the branch holdingregister for pre-decode taken branch instruction and written into thearray for pre-decode non-taken branch instruction.

ICBCOL1(3:0)--Internal and output to decode units, indicates the columnof the first branch target in the ICNXTBLK for the lines of instructionsbeing fetched. This signal must be greater or equal to STARTPTR. Thissignal qualifies the BTAG1 in the decode units.

ICBCOL2(3:0)--Internal and output to decode units, indicates the columnof the second branch target in the ICNXTBLK for the lines ofinstructions being fetched. This signal must be greater or equal toSTARTPTR. This signal qualifies the BTAG2 in the decode units.

BVAL1--Internal and output to decode units to qualify BTAG1, indicatesthe first branch target is greater or equal to STARTPTR.

BVAL2--Internal and output to decode units to qualify BTAG2, indicatesthe second branch target is greater or equal to STARTPTR.

BTAKEN1(1:0)--Output to decode units and ICFPC indicates a predictedtaken branch instruction from PTAKEN, BVAL1. Bit 0 is the last line andbit 1 is new line.

BTAKEN2(1:0)--Output to decode units and ICFPC indicates a predictedtaken branch instruction from PTAKEN, BVAL2. Bit 0 is the last line andbit 1 is new line.

SINDEX(14:0)--Output indicates the successor index from branchprediction, selected by PTAKEN, BVAL1, and BVAL2.

BTAG1(3:0)--Output indicates the position of the first target branchinstruction with respect to the global shift register in case of branchmis-prediction.

BTAG2(3:0)--Output indicates the position of the second target branchinstruction with respect to the global shift register in case of branchmis-prediction.

BTAG1N(3:0)--Output indicates the position of the first target branchinstruction for a new line with respect to the global shift register incase of branch mis-prediction.

BTAG2N(3:0)--Output indicates the position of the second target branchinstruction for a new line with respect to the global shift register incase of branch mis-prediction.

ICPWAY(2:0)--Internal to select the index from way prediction. Thebranch prediction and targets are used in the ICNXTBLK to access thenext line. This signal selects one of the eight way of successor indexesto ICFPC. The ICFPC needs to compare this signal against TAGHIT tovalidate the way prediction.

ICPTAR(1:0)--Output to ICFPC indicates which branch target to use toaccess the cache in the next cycle. 00--sequential, 01--return stack,10--first branch target, and 11--second branch target. Bit 0 selects oneof the two successor indexes before the way prediction. The ICFPC needsto compare this signal against the actual taken/non-taken branchprediction. For return stack, the decode unit should detect the returninstruction.

WINDEX(14:0)--Output to ICFPC indicates the successor index from wayprediction.

DISPBRN1(1:0)--Input from ICALIGN indicates the first predicted branchinstruction dispatched to a decode unit. This signal is the OR ofICxBRNB1. Bit 0 is for the first 8-byte and bit 1 is for the second8-byte of the line if NEWLINE(1) is set for the second 8-byte.

DISPBRN2(1:0)--Input from ICALIGN indicates the second predicted branchinstruction dispatched to decode unit. This signal is the OR ofICxBRNB2. Bit 0 is for the first 8-byte and bit 1 is for the second8-byte of the line if NEWLINE(1) is set for the second 8-byte.

BSTRD--Input from TAP indicates to read the array and compare to set theresult.

BSTWR--Input from TAP indicates to write the array from input registers.

BSTDIN--Input from TAP indicates the test pattern to the inputregisters. The input can be from the TDI pin or normal burn-in patterns.

FLUSHON--Input from TAP indicates flushing register mode, the resultlatch should use BSTDIN instead of the compare input for flushing theresult registers.

BSTSHF1--Input from TAP indicates shifting of the master latch ofregisters.

BSTSHF2--Input from TAP indicates shifting of the slave latch ofregisters.

BSTFALSE--Input from TAP indicates to invert the test pattern.

PORTSEL--Input from TAP indicates to select the second dual port.

BSTNBOUT--Output to ICTAGV indicates the result of the data chain.

ICFPC

For continuous instruction addresses, the Icache would be required torequest for that IAD bus send the address to the MMU for re-translationbefore going external. Moving the instruction address latch andincrementor to the ABI block, the instruction address by default can beon the external address bus which can increase the performance of theDRAM access in burst mode. With this change, continuous instructionaddress request would be relatively simple since the Icache can make arequest directly to the BIU without any handshaking for address. With ataken branch, the instruction address latch in ABI will be invalidated,and a new address must be sent to the MMU. The instruction address latchmust have a detection for the page-boundary overflow (NAND gates for thelower bits of the address).

This block contains the current fetch PCs to each array, the PCincrementor for sequential access, and the branch holding addressregister of mis-predicted block for updating of the ICNXTBLK. The branchholding address register is always be compared to the PC to forward thebranch data instead of reading from the ICNXTBLK. The ICFPC should keeptrack of the PCs for all instructions from fetching until all theinstructions are valid and all the possible branch instructions areresolved and issued to the decode units. Status bits are needed toindicate the branch targets and return instruction prediction. TheICPDAT provides the information of each valid 8-byte sending to ICALIGNand the ICALIGN provides the information as an instruction starting on anew 8-byte block is dispatched to the decode unit. The instruction mustbe validated by proper start and end bits, and the RETURN andunconditional jump instructions are detected. These are the possiblecases:

1. Invalid instruction must be pre-decoded again:

    ______________________________________                                        Clock     Latch 1 Latch 2     Operation of PC0                                ______________________________________                                        1         PC0     --          read                                            2         PC1     PC0         invalid instruction                             3         PC2     --          restart                                         ______________________________________                                    

2. Branch prediction is not the same as way prediction:

    ______________________________________                                        Clock     Latch 1 Latch 2     Operation of PC0                                ______________________________________                                        1         PC0     --          read                                            2         PC1     PC0         mis-predict                                     3         PC2     --          restart                                         ______________________________________                                    

3. Invalid long instruction (more than two 8-byte blocks) must bepre-decoded again:

    ______________________________________                                        Clock  Latch 1   Latch 2 Latch 3 Operation of PC0                             ______________________________________                                        1      PC0       --      --      read                                         2      PC1       PC0     --      long instruction                             3      PC2       PC1     PC0     invalid instruction                          4      PC0       --      --      restart                                      ______________________________________                                    

4. RETURN branch is mis-predicted in early decoding (compare to wayprediction):

    ______________________________________                                        Clock Latch 1 Latch 2  Latch 3                                                                             Latch 4 Operation of PC0                         ______________________________________                                        1     PC0     --       --    --      read                                     2     PC1     PC0      --    --      scan & mux                               3     PC2     PC1      PC0   PC0     early decode                             4     PC3     PC2      PC1   PC0     RET mis                                  5     PC0     --       --    --      restart                                  ______________________________________                                    

A timing diagram for updating of ICNXTBLK from way misprediction isshown as FIG. 17.

Linear/Logic Address Calculation

The ICACHE uses linear address while the decode units use logicaladdresses. The code segment register is included in the ICFPC. Thetranslation is either for 16-bit addressing or 32-bit addressing, andeither real or protected mode. The code segment register includes:

Segment selector (15:00)

Base Address 15:00, Segment Limit 15:00

Base Address 31:24, Control, Limit 19:16, Control, Type, Base Address23:16

The Icache is accessed by using linear address while execution of branchinstruction uses logical address. On Processor 500, these functionsshould be done in the ICFPC: calculating the linear address for a newfetch PC, checking for code segment limits, and calculating the logicalPC address for dispatching new instructions to IDECODE.

For a mis-predicted branch, the Icache needs to write the branch holdingregister into the ICNXTBLK before accessing the cache with a new fetchPC. The linear address can be calculated in the Icache without anypenalty in performance. The functional unit should send new logicaladdress PC to the Icache directly.

Since Processor 500 does not have a BYTEQ, 4 X86 instructions arescanned and dispatched directly to decode units; the PC of dispatchedinstructions is known for each clock cycle. The code segment limitschecking should be done in the ICFPC and a violation status can be sentwith the instruction to the decode unit. The ICFPC should keep both thelogical and linear PC addresses for the 4 pipeline stages from fetchingto muxing of instructions to decode units, the logical address is sentto the decode unit without any calculation.

Return Stack

The RETURN instruction should be detected in early decoding and fetchthe next PC from return stack (the RETURN instruction will not be in theICNXTBLK). It should be individually decoded in the early decode units.Similarly, the CALL instruction should also be detected in the earlydecode units to update the return stack. The CALL instruction pushes PCof the next instruction onto the stack which is the concatenation of theline PC and the ICPOSx(3:0). The return stack is a last-in-first-out(LIFO) stack. The return stack is implemented with a pointer to the topof the stack, and the pointer increases as a new entry is allocated forthe CALL instruction. When the return stack is full, the stack isshifted down by one for the new CALL instruction, and the pointer shouldbe at top of the stack. In another case, when both the call and returninstructions of the same entry in the return stack are retired, theentry can be removed by shifting all entries down by one and the pointerdecreases by one. The return stack must be recovered from branchmis-prediction. The mis-predicted RETURN instruction is difficult tohandle, there are three possible cases. In the application program, theRETURN instruction can be fake which causes the return stack to beforever mis-predicted, the subroutine changes the return target in thestack pointer before executing the RETURN instruction, and thesubroutine uses JMP instruction instead of RETURN instruction. Forsimplicity, the mis-predicted RETURN instruction will cause the returnstack to be invalidated. Another condition for invalidating the returnstack is from the CMASTER invalidating the Icache. For RETURNinstruction after the mis-predicted branch instruction, the return stackshould be able to recover. The current return stack is eight buffers,each buffer contains the call branch tag from the global shift registerwhen the CALL instruction is dispatched, the return PC, the returnbranch tag, the call valid bit, the return valid bit, and the retiredcall valid bit. The best way to understand the recovery procedure is bythe example of this sequence of instructions:

Jump1, Ret2 C, Call3 D, Jump4,

Ret5 D, Call6 E, Call7 F, Ret8 F,

Ret9 E, Ret10 B, Call11 G.

Beginning return PCs in the return stack: C, B, A.

    ______________________________________                                        Return PC                                                                              Call Tag   Return Tag                                                                              CV    RV  RCV                                   ______________________________________                                        C        0          --        1     0   0                                     B        0          --        1     0   1                                     A        0          --        1     0   1                                     ______________________________________                                    

The return stack at Jump4 instruction:

    ______________________________________                                        Return PC                                                                              Call Tag   Return Tag                                                                              CV    RV  RCV                                   ______________________________________                                        D        3          --        1     0   0                                     C        0          2         1     1   0                                     B        0          --        1     0   1                                     A        0          --        1     0   1                                     ______________________________________                                    

The return stack at Call7 F instruction:

    ______________________________________                                        Return PC                                                                              Call Tag   Return Tag                                                                              CV    RV  RCV                                   ______________________________________                                        F        7          --        1     0   0                                     E        6          --        1     0   0                                     D        3          5         1     1   0                                     C        0          2         1     1   0                                     B        0          --        1     0   1                                     A        0          --        1     0   1                                     ______________________________________                                    

The return stack at the end of the sequence:

    ______________________________________                                        Return PC                                                                              Call Tag   Return Tag                                                                              CV    RV  RCV                                   ______________________________________                                        G        11         --        1     0   0                                     F        7          8         1     1   0                                     E        6          9         1     1   0                                     D        3          5         1     1   0                                     C        0          2         1     1   0                                     B        0          10        1     1   1                                     A        0          --        1     0   1                                     ______________________________________                                    

If Jump 1 is mis-predicted, then Tag 1 is used for recovery. The CallTag in the recovery stack should be less than Tag 1 and Return Tagshould be greater than Tag 1 for valid in the return stack. Entries G,F, E, and D should invalidate all valid bits. Entries C and B shouldreset the return valid bits.

If Jump 4 is mis-predicted, then Tag 4 is used for recovery. Entries G,F, and E should invalidate all valid bits. Entries B and D should resetthe return valid bits. There is no change for entry C.

If Call3 D is mis-predicted, then Tag 3 is used for recovery. Entries G,F, and E should invalidate all valid bits. Entries D, C, and B shouldreset the return valid bits. A new return PC is latched for entry D.

If Call7 F is mis-predicted, then Tag 7 is used for recovery. Entry Gshould invalidate all valid bits. Entries E and F should reset thereturn valid bits. A new return PC is latched for entry F.

If Call 11 G is mis-predicted, then Tag 11 is used for recovery. A newreturn PC is latched for entry G.

Any mis-predicted return instruction invalidates all valid bits in thereturn stack. The call instruction should retire before the returninstruction in the same entry. As the return instruction is retired theentry can be eliminated from the return stack and all the entries can beshifted down by one.

The INT and IRET are similar to the CALL and RETURN instructions. Thereturn stack needs to keep the PC and statuses for the INT and IRET. Itis more often that the interrupt routine is from asynchronous interruptand trap. The IRET executes without the previous INT. To avoidinvalidating the entire return stack on mis-predicted IRET, two morestatus bits are needed. The IXC bit indicates that the entry is from INTor CALL instruction. The IRET pops only the top entry with IXC bit set,and the RETURN pops only the top entry with IXC bit reset. The ISTARTbit indicates the starting point for INT routine. In the case of amis-prediction on the IRET instruction, entries with ISTART bit set willbe purged. The entries before the INT should not be affected by themis-prediction from the interrupt routine. A diagram of the return stackset up is shown as FIG. 18.

ICFPC ORGANIZATION

The possible sources for index of ICSTORE are:

Incrementor of sequential address.

Refresh of current index.

Refresh the previous index because of confused instruction in decode.

Successor index of taken branch from way predictor of ICNXTBLK.

Return instruction target from way predictor of ICNXTBLK.

Corrected index of taken branch from branch prediction of ICNXTBLK.

Predicted taken branch from ICPRED.

Branch mis-prediction or Read-after-Write dependency flush from FIROB.

Special register reading/writing.

Build-In Self-Test counter.

The possible sources for index of ICTAGV are:

Incrementor of sequential address.

Refresh of current index.

Refresh the previous index because of confused instruction in decode.

Next block address to check the cache during pre-fetching.

Successor index of taken branch from way predictor of ICNXTBLK.

Return instruction target from way predictor of ICNXTBLK.

Corrected index of taken branch from branch prediction of ICNXTBLK.

Branch mis-prediction or Read-after-Write dependency flush from FIROB.

Predicted taken branch from ICPRED.

CMASTER new mapping for current tag miss.

Special register reading/writing.

Build-In Self-Test counter.

The possible sources for index of ICPDAT are:

Incrementor of sequential address for reading (same as ICSTORE).

Refresh of current index.

Refresh the previous index because of confused instruction in decode.

Delay of sequential address for writing of pre-decode data.

Successor index of taken branch from way predictor of ICNXTBLK.

Return instruction target from way predictor of ICNXTBLK.

Corrected index of taken branch from branch prediction of ICNXTBLK.

Branch mis-prediction or Read-after-Write dependency flush from FIROB.

Predicted taken branch from ICPRED.

Special register reading/writing.

Build-In Self-Test counter.

The possible sources for index of ICNXTBLK are:

Incrementor of sequential address.

Refresh of current index.

Refresh the previous index because of confused instruction in decode.

Delay of sequential address for writing of pre-decode data.

Successor index of taken branch from way predictor of ICNXTBLK.

Return instruction target from way predictor of ICNXTBLK.

Corrected index of taken branch from branch prediction of ICNXTBLK.

Branch mis-prediction or Read-after-Write dependency flush from FIROB.

Predicted taken branch from ICPRED.

Branch holding address register.

Special register reading/writing.

Build-In Self-Test counter.

The ICFPC block also includes the code segment register, the PCincrementor address, the branch holding address register and comparator,and the subtractor for calculation of logical address. The code segmentregister includes the base for logical address calculation and the limitfor segment violation. The PC incrementor has two parts: the indexincrementor and the tag-address incrementor; the tag-address incrementoris used only when the index incrementor is overflowed. It is much fasterto break up the PC incrementor into two parts.

ICFPC should keep track of all the way selection in all the arrays. Fromreading the array, the predicted way and the TAGHITs should match, andthe HIT way should be kept with the PC and branch tags. For writing thearray, the selected way is from the CMASTER's pseudo random wayreplacement algorithm or from the branch mis-prediction. For the specialregister read/write, the selected way is from the SRB.

An 11-bit counter is implemented in this block for BIST, the counterfakes counting down by exclusive-or the output. See the section on BISTand ATPG.

Signal list

REFRESH2--Input from Idecode indicates current line of instructions willbe refreshed and not accept new instructions from Icache.

NEWLINE(1:0)--Input from ICPDAT indicates the first (bit 0) or thesecond (bit 1) 8-byte is the starting of a new line. The byte positionsof instructions sending to the decode units should be adjustedaccordingly.

HLDISP(1:0)--Input from ICALIGN indicates the first (bit 0) and/or thesecond (bit 1) 8-byte of the current line has been dispatched to decodeunits.

NEWBLK--Input from ICCNTL to read new sequential block.

NXTBLK--Input from ICCNTL to check next sequential block duringpre-fetching.

INVPTR(3:0)--Input from ICPDAT indicates the start pointer of theinvalid instruction which needs to be pre-decoded.

INVINST--Input from ICPDAT indicates the invalid instruction which needsto be pre-decoded.

ICPTAR(1:0)--Input from ICNXTBLK indicates which branch target to use toaccess the cache in the next cycle. 00--sequential, 01--return stack,10--first branch target, and 11--second branch target. Bit 0 selects oneof the two successor indexes before the way prediction. The ICFPC needsto compare this signal against the actual taken/non-taken branchprediction.

WINDEX(14:0)--Input from ICNXTBLK indicates the successor index from wayprediction.

RETPC(31:0)--Internal indicates the PC address from the top of thereturn stack for fast way prediction.

CALLDEC(3:0)--Input from ICALIGN indicates the CALL instruction isdetected, the return stack should be updated with the PC address ofinstruction after CALL. The line must be taken.

NORETB--Input from ICALIGN indicates no RETURN instruction is detectedfor the current line. This signal responds to RETPRED. The ICFPC shoulduse the branch prediction of BTAKEN to fetch the next line.

RETDEC(3:0)--Input from ICALIGN indicates a RETURN instruction isdetected for the way prediction to update if no RETURN was predicted.The ICFPC needs to keep track of the branch instructions for 4 lines andrestores the global shift register in case of mis-predicting the RETURNinstruction in Idecode.

BTAKEN1(1:0)--Input from ICNXTBLK indicates a predicted taken branchinstruction from PTAKEN, BVAL1. Bit 0 is the last line and bit 1 is newline. SINDEX should be used for accessing the arrays.

BTAKEN2(1:0)--Input from ICNXTBLK indicates a predicted taken branchinstruction from PTAKEN, BVAL2. Bit 0 is the last line and bit 1 is newline. SINDEX should be used for accessing the arrays.

SINDEX(11:0)--Input from ICNXTBLK indicates the correct successor indexfrom branch prediction, selected by PTAKEN, BVAL1, and BVAL2.

PDJMP--Input from ICPRED indicates the current instruction is a branchinstruction. The branch holding register should be setup untilpre-decoding of the whole line is completed. If there is a taken branchinstruction, the new branch holding register must wait for MWAY and notupdate the ICNXTBLK until the next opportunity.

PDTAKEN--Input from ICPRED indicates the current instruction is takenbranch (the taken branch target can be calculated). The predictorcounter in the branch holding register should be setup untilpre-decoding of the whole line is completed. The successor index and wayassociative will be sent to the ICNXTBLK by ICFPC. The next index forthe array is from PTARGET

PDRETB--Input from ICPRED indicates the current instruction is RETURNinstruction. The ICFPC should use the return branch address for fetchingthe next line.

PTARGET(31:0)--Input from ICPRED indicates the branch target for currentbranch instruction. The successor index is latched until pre-decoding ofthe whole line is completed. If the branch is taken, the way calculationis done in ICFPC and latched until branch mis-predictions or externalfetch is started.

ICMBWR--Input from ICCNTL indicates starting of external fetch, thebranch holding register should be written into the ICNXTBLK in the nextcycle. The array index is from BINDEX.

BRNMISP--Input from FIROB indicates a branch mis-prediction. The Icachechanges its state machine to access a new PC and clears all pendinginstructions. This signal is one cycle ahead of the transferring of thenew target. The branch holding register should be written into theICNXTBLK array in next cycle. The array index is from BINDEX.

LINDEX(11:0)--Internal from branch holding register indicates theprevious branch to be written into ICNXTBLK.

UPDFPC--Input from FIROB indicate that a new Fetch PC has been detected.This signal accompanies the FPC for the Icache to begin access the cachearrays.

FPC(31:0)--Input from FIROB as the new PC for branch correction path.

BPC(11:0)--Input from FIROB indicates the PC index and byte-pointer ofthe branch instruction which has been

mis-predicted for updating the ICNXTBLK. This index must be compared tothe array index for exact recovery of the global shift register.

BRNTAKEN--Input from FIROB indicate the mis-predicted branch status.This signal must be gated with UPDFPC.

BRNTAG(3:0)--Input from FIROB indicates the branch tag in the globalshift register to restore the previous branch prediction information andthe branch index address. The mis-predicted will be latched into thebranch holding register. The branch index of the branch holding registeris implemented in the ICFPC.

BINDEX(11:0)--Internal, indicates the previous branch index, the branchindexes are address by the branch tag with similar implementation as theglobal shift register in the ICNXTBLK.

FPCTYP--Input for FIROB indicates the type of address that is beingpassed to the Icache.

L2₋₋ IC₋₋ INV(1:0)--Input from CMASTER to clear the LV bits.

LS₋₋ CS₋₋ WR--Input from LSSEC to write the new code segment from SRBB.Two cycles are needed.

MVICAR--Output to ICCNTL indicates move to/from special registerinstruction is for Icache array. The ICCNTL steals one cycle from normaloperation.

SRRDXWR--Output to ICCNTL indicates move to/from special registerinstruction is read or write.

SRB₋₋ VAL--Input from SRB indicates a special register instruction is onthe SRBB.

ICTAR₋₋ VAL--Output to SRB indicates completion of the special registerinstruction, for read the data is on the SRBB.

SRBB(31:0)--I/O from SRB indicates the special register address for theindexing the arrays, passing to ICSRBB, and to access the code segmentregister.

ICSRBB(31:0)--I/O passing from SRBB for internal access to the arrays. Anumber of bits will pass to the ICCNTL for decoding which array.

L2₋₋ IC₋₋ ALIAS--Input from CMASTER indicates the instruction is in theIcache with different mapping. The CMASTER provides the way associative,new supervisor bit, and new tag address. The LV will be set in thiscase. All arrays will be read from MCOL except for ICTAGV which willwrite in the new tag.

PFREPLCOL(2:0)--Input from CMASTER indicates the way associative forwriting of the ICTAGV.

PREDEN--Input from ICCNTL to enable the pre-decoding of instruction.

GETNEWL--Input from ICPRED indicates that pre-decoding has reached theend of the line or encountered a taken branch instruction, a new line ofinstructions is needed in the next cycle. This signal controls the indexfor ICSTORE and ICTAGV.

ENDLINE--Input from ICPRED indicates that pre-decoding has completed forthe last instruction of the line (if the instruction crosses over to thenext line, this instruction should complete in pre-decoding) orencountered a taken branch instruction, the pre-decode data should bewritten into the array. This signal controls the index for ICPDAT andICNXTBLK.

ENDINST--Input from ICPRED indicates that pre-decoding is completed forthe current instruction. The pre-decode bits should be set at this time.The ICFPC uses this signal and PENDB to generate a new STARTPTR.

PDJMP--Input from ICPRED indicates the current instruction is a branchinstruction. The branch holding register should be setup untilpre-decoding of the whole line is completed. If there is a taken branchinstruction, the new branch holding register must wait for BWAY and notupdate the ICNXTBLK until the next opportunity.

PDTAKEN--Input from ICPRED indicates the current instruction is takenbranch (the taken branch target can be calculated). The predictorcounter in the branch holding register should be setup untilpre-decoding of the whole line is completed. The successor index and wayassociative will be sent to the ICPRED by ICFPC.

DISPBRN1(1:0)--Input from ICFPC indicates the first predicted branchinstruction dispatched to decode unit. This signal is the OR ofICxBRNB1. Bit 0 is for the first 8-byte and bit 1 is for the second8-byte of the line if NEWLINE(1) is set for the second 8-byte.

DISPBRN2(1:0)--Input from ICFPC indicates the second predicted branchinstruction dispatched to decode unit. This signal is the OR ofICxBRNB2. Bit 0 is for the first 8-byte and bit 1 is for the second8-byte of the line if NEWLINE(1) is set for the second 8-byte.

STARTPTR(3:0)--Output indicates the starting byte position forpre-decoding of the instruction in the IB(127:0).

ISADD(11:2)--Output to ICSTORE indicates the address of instruction toaccess the array. Bits 11:5 are for the row decoder, bits 4:2 are forcolumn select. For normal operation, bits 11:4 is used to read/write16-byte only. For special register operation, bits 11:2 is used toread/write 4-byte.

IPADD(11:2)--Input from ICFPC indicates the address of instruction toaccess the array. Bits 11:6 are for the row decoder, bits 5:2 are forcolumn select. For normal operation, bits 11:4 is used to read/write16-byte only. For special register operation, bits 11:2 is used toread/write 4-byte.

ITADD(11:4)--Output to ICTAGV indicates the address of instruction toaccess the array. Bits 11:5 are for the row decoder, bit 4 is for columnselect.

IVADD(11:4)--Output to ICTAGV indicates the address of instruction toaccess the LV array. Bits 11:5 are for the row decoder, bit 4 is forcolumn select.

INADD(11:3)--Input from ICFPC indicates the address of instruction toaccess the array. Bits 11:5 are for the row decoder, bit 4 is for columnselect. This index should be delayed by 3 cycles to write data for theway prediction bits.

SETSEL(7:0)--Output to arrays indicates which set to read, no tagcompare is needed.

BWAY(2:0)--Output to ICNXTBLK indicates the way associative for thecurrent taken branch instruction for the branch holding register and wayprediction.

BTAR(1:0)--Output to ICNXTBLK indicates the target of the current linefor way prediction. This signal is three cycles after the current lineaccess.

ICLIMIT(19:0)--Output to decode units indicates the limit of the codesegment register for segment violation.

ICPC1TAR--Output to Idecode indicates is ICPC1 a branch target of aprevious instruction which is a predicted taken branch instruction.

ICPC2TAR--Output to Idecode indicates is ICPC2 a branch target of aprevious instruction which is a predicted taken branch instruction.

ICPC1(31:0)--Output to Idecode Output to Idecode indicates the currentline PC of the first instruction in the 4 issued instructions to passalong with the instruction to FIROB.

ICPC2(31:0)--Output to Idecode indicates the current line PC of a secondinstruction which cross the 16-byte boundary or branch target in the 4issued instructions to pass along with the instruction to FIROB.

RETPRED--Output to Idecode indicates the current prediction of thereturn instruction of the fetched line. The return instruction must bedetected in the current line of instruction or the Icache must bere-fetched from a new line.

BSTRUN--Input from TAP indicates to start the BIST.

BSTRD--Input from TAP indicates to read the array and compare to set theresult.

BSTWR--Input from TAP indicates to write the array from input registers.

BSTRST--Input from TAP indicates to reset the counter.

BSTINCR--Input from TAP indicates to increment the counter.

UPDOWN--Input from TAP indicates counting up or down.

BSTAMSB--Output to TAP indicates maximum count for dual port arrays.

MAXADDR--Output to TAP indicates maximum index counter.

ICPRED

This block pre-decodes the instructions as they come in from theexternal memory or from the Icache if the start/end bits are confused.On Processor 500, the ICPRED is connected to the IB(127:0) to read theinstructions from either pre-fetch buffer or Icache. For externallyfetched instruction, the ICPRED starts from the fetched byte position.The ICPRED latches the instructions in the second ICLK as they are sentto the decode units. If the start/end bits are confused, then the ICPDATsends the byte position of the invalid instruction to the ICPRED forpre-decoding. All pre-decode data before the confused instruction shouldremain in the ICPDAT array. The pre-decoding stops on two conditions: ataken branch is detected or valid start byte is detected after the endbyte from pre-decoding; this condition should be detected in the ICPDAT.The ICPRED pre-decodes one instruction per two clock cycles plus anextra clock cycle for prefix. The pre-decode information includes thestart-byte, end-byte, function-byte, and the branch predictioninformation. The difference in pre-decode of instructions are:

Fast-path instructions should have any number of prefix except forstring prefixes which is an MROM instruction. One extra clock cycle inthe pipeline is needed for instructions with more than four prefixes.

A few instructions use two dispatched positions. The SIB instructionalso needs two dispatched positions. Any double-dispatched instructionswith SIB will go to MROM.

The first functional bit, if set, indicates fast-path instruction, elseit is MROM instruction. The Processor 500 instruction set includes:

PUSH

POP

CALL/RETURN near

LEA

JUMP cc/unconditional

8/32-bit operations

MOVE reg/reg reg/mem

ALU operations reg/mem reg/reg (excluding the RCR and RCL instructions).

Decode for CALL, INT, and Unconditional JUMP to set the predictor andthe bimodal counters to the saturated value in the ICNXTBLK. The branchtarget information is kept in the branch holding register for futureupdating. The ICPRED calculates the target address for the UnconditionalJump if possible. Otherwise, fetching is stalled until the targetaddress is calculated in decoding or executing. ICFPC must have an adderand ICPRED must send the offset to calculate the target address.

The RETURN and IRET instructions are pre-decoded to access the returnstack and set the way prediction array.

Decode for Conditional JUMP is needed to keep it in the ICNXTBLK and forglobal branch prediction. If the target address of the conditional JUMP(and LOOP) can be calculated and the branch jumps backward, then it ispredicted taken. Since backward branch is mostly taken and the adder isavailable to calculate the target address, the conditional branch shouldbe predicted taken. The taken branches have the higher priority tooccupy the branch targets. Conditional branch has higher priority tooccupy two branch targets in the ICNXTBLK than CALL or UnconditionalJUMP with 8-bit displacement linear address. The decoding for allbranches are needed to set the global shift register. If the targetaddress is a simple calculation, the decode units should calculate thetarget address and jump to new block.

The encoding of the functional bits for fast-path instruction are asfollows:

    ______________________________________                                        Byte number 01234 Type                                                        ______________________________________                                        Start bit   1000       MROM, no prefix                                        Func bit    0111       opcode is at first byte.                               Start bit   1000       MROM, one prefix                                       Func bit    0011       opcode is at second byte                               Start bit   10000      MROM, three prefixes                                   Func bit    00001      opcode is at fourth byte                               Start bit   1000       Fast path, no prefix                                   Func bit    1000       opcode is at first byte                                Start bit   1000       Fast path, one prefix                                  Func bit    1100       opcode is at second byte                               Start bit   10000      Fast path, three prefix                                Func bit    11110      opcode is at fourth byte                               ______________________________________                                    

Along with the start bit, if the functional bit is 0, the instruction isMROM, if the functional bit is 1, the instruction is fast path. Theencoding for the functional bits of MROM instruction is inverted that ofthe fast path instruction. If the first functional bit is 0, allfunctional bits is inverted before qualifying with the valid bits forprefix and opcode decoding. In addition, the functional bits should alsobe set for SIB-byte and for the immediate field. The detection of theSIB-byte instruction is necessary to generate 2-dispatch positionsinstruction.

The ICPRED controls the reading of the next sequential line from ICSTOREto IB(127:0) and writing of the pre-decode data into the ICPDAT andICNXTBLK. GETNEWL is used to read the next sequential block fromICSTORE, and ENDLINE and ENDINST are used to write pre-decode data intothe ICPDAT and ICNXTBLK. If the split-line instruction (instructionstarted at the end of the first line and end in the next line) is also apredicted taken branch, then ICCNTL and ICPRED should provide control towrite predecode data into the ICPDAT array in an extra cycle. ICPREDprovides the start and end pointers as completion of pre-decoding ofeach instruction along with ENDINST. The functional bits are kept inICPRED until completion of the line. ICPRED must continuously latch thenew line from IB(127:0) in case of confused instruction, all functionalbits should be cleared before pre-decoding.

The pre-decode instruction should be sent directly to decode unit 0. Theinstruction has to go through early decoding before dispatching to thedecode unit. During pre-decoding all the controls of sending the correctinstruction from IB(127:0) to IB1(191:0) should be directly from ICPRED.The ICPRED controls the muxing between the 2 buses and the ICALIGN usesPDSHF(4:0) to read the instruction from the 8-byte of IB1 for the validopcode, MODRM, and SIB bytes.

Upon detection of MROM instruction, the ICPDAT muxes the STARTPTR(3:0)to the MROM for reading of instruction from IB(127:0). The ICPRED mustcontinue to decode instructions and fill up the bus IB1(191:0) of up to4 instructions per 8-byte block. The ICALIGN keeps PDSHF(4:0) inICnVAL(7:0) of each 8-byte block. Similar to ICPDAT, the informationfrom the next stage in ICALIGN and MROM pipeline should be conveyed tothis block. The pre-decoding is stalled if ICALIGN or MROM is full.

ICPRED Organization

The ICPRED includes three blocks: one block (ICPREFIX) decodes theprefix, one block (ICDECINS) decodes the instruction, and one block(ICPREINS) sets up the pre-decode data. The ICPREFIX decodes up to 4prefix bytes per clock cycle. The ICPREFIX is similar to the prefixdecoding in the ICALIGN, and the same logic should be used. The ICPREFIXshould combined the prefixes for dispatching to decode unit; this is toavoid an extra cycle if there are more than 4 prefixes. The ICDECINSaccepts prefix status and 3 instruction bytes, the opcode, the MODRM,and the SIB. In the first cycle, no prefix is assumed. If prefix isdetected in the ICPREFIX, the ICDECINS will restart the decoding ofinstruction with new prefix status and 3 new instruction bytes. If moreprefixes are detected in subsequent cycle, the ICDECINS will restartwith new information. After the decoding, the pre-decode informationwill be sent to the decode unit 0 and the latch in the ICPDAT, theICALIGN will dispatch the instruction and pre-decode data to the firstdecode unit. The byte pointer moves to the next instruction and theprocedure continues until the whole line is completed. The writing ofthe ICPDAT will be done when the whole line is decoded. The ICDECINSalso decodes for branch instructions and sets up the two targets in theICNXTBLK. The ICDECINS sends control to ICFPC for calculating the simpletaken branch addresses: PC+displacement. The ICPRED includes locallatches of 8 bytes for instruction which can wrap around to the nextline. For instructions which are longer than 15 bytes, an exception isasserted to ICCNTL and decode units. The list of Processor 500 fast pathinstructions can be found below. The outputs of the ICPREFIX andICDECINS are fed to ICPREINS for analyzing and setting up the pre-decodedata. A diagram showing the ICPRED set up is shown as FIG. 19.

Signal List

IB(127:0)--Input from ICSTORE indicates the line of instructions fromthe array or pre-fetch buffer for pre-decoding. A latch should beimplemented for unaligned instructions. Data only changes on validGETNEWL and PBVAL.

STARTPTR(3:0)--Input from ICFPC indicates the starting byte position forpre-decoding of the instruction in the IB(127:0).

CS32X16--Input from LSSEC indicates operand and address size from the Dbit of the segment descriptor of the code segment register. If set,32-bit, if clear, 16-bit.

PREDEN--Input from ICCNTL to enable the pre-decoding of instruction.

PBVAL--Input from ICSTORE indicates a valid line of instruction fromexternal fetch. If GETNEWL is high, the pre-fetch buffer will put avalid line of instruction on the IB(127:0) in early next cycle. ICPREDshould use both PBVAL and GETNEWL to read valid instruction byte on IB.

GETNEWL--Output indicates that pre-decoding has reached the end of theline or encountered a taken branch instruction, a new line ofinstructions is needed in the next cycle. This signal is also assertedat the beginning of PREDEN.

ENDLINE--Output indicates that pre-decoding has completed for the lastinstruction of the line (if the instruction crosses over to the nextline, this instruction should complete in pre-decoding) or encountered ataken branch instruction, the pre-decode data should be written into thearray.

ENDINST--Output indicates that pre-decoding is completed for the currentinstruction. The pre-decode bits should be set at this time. This signalshould also be sent along with ENDLINE for partially decode instruction.

PENDIB(4:0)--Output to ICPDAT indicates the end byte position of thecurrent instruction. The end bit is set and latched until pre-decodingof the whole line is completed. Bit 4 indicates if the end bit is on thenext line.

PLFUNCB(15:0)--Output to ICPDAT indicates the functional bits of currentline of instructions. ICPDAT uses ENDLINE to OR these bits previouspre-decode data and write into the array. The ICPRED must keep all thefunctional bits for all the pre-decoded instruction in the line.

PDMROM--Output to ICDPAT and ICALIGN indicates the current instructionis MROM. The MROM instruction may take two cycles to read the PENDB andPFUNCB.

PIFUNCB(2:0)--Output to ICALIGN indicates the functional bits forcurrent instruction. Only three bytes (opcode, MODRM, and SIB) areneeded for early decode of the instruction. The prefixes are encodedinto one single byte to the decode unit 0.

PDVAL(10:0)--Output to ICALIGN indicates the valid bytes for pre-decodeinstruction to decode unit 0. This signal is to validate the bytes indecode. The maximum length of the instruction is 11 bytes.

PDSHF(5:0)--Output to ICALIGN indicates which bytes to mux to earlydecoding. The opcode byte must always be in the first 8-byte ofIB1(191:0). The ICPRED must put the current line into the IB1(191:0) formuxing. The upper 2 bits to select which 8-byte block, the ICALIGN canaccumulate the pre-decode instructions while waiting for the MROMinstructions.

PD8BSEL(5:0)--Output to ICALIGN controls the muxes from IB(127:0) toIB1(191:0). Bit 5:3 is for the upper 8-byte IB(127:64) and bit 2:0 isfor the lower 8-byte IB(63:0). 001 is for muxing to the first 8-byteIB1(63:0), 010 is for muxing to the second 8-byte, and 100 is for muxingto the third 8-byte.

PDPREF(7:0)--Output to ICALIGN and then to decode unit 0 or MROMindicates the encoded prefix byte.

PDJMP--Output indicates the current instruction is a branch instruction.The branch holding register should be setup until pre-decoding of thewhole line is completed. If there is a taken branch instruction, the newbranch holding register must wait for MWAY and not update the ICNXTBLKuntil the next opportunity.

PDTAKEN--Output indicates the current instruction is taken branch (thetaken branch target can be calculated). The predictor counter in thebranch holding register should be setup until pre-decoding of the wholeline is completed. The successor index and way associative will be sentto the ICNXTBLK by ICFPC.

PDRETB--Output to ICFPC indicates the current instruction is RETURNinstruction. The ICFPC should use the return branch address for fetchingthe next line.

INST₋₋ LEGAL--Output to ICCNTL indicates the current instruction is alegal X86 instruction, this signal should be qualified with ENDINST toindicate illegal instruction to the decode unit.

INST₋₋ 2₋₋ LONG--Output to ICCNTL indicates the current instruction ismore than 15 bytes, this signal should be qualified with ENDINST toindicate long instruction to the decode unit.

PTARGET(31:0)--Output to ICFPC indicates the branch target for currentbranch instruction. The successor index is latched until pre-decoding ofthe whole line is completed. If the branch is taken, the way calculationis done in ICFPC and latched until branch mis-predictions or externalfetch is started.

ICALIGN

An extra pipeline stage is added on Processor 500 for early decoding ofprefixes and generating two dispatched positions for a few instructions.To gain another half clock cycle for early decoding and dispatching ofinstructions to the decode units, the operand, flag, and un-conditionaljump instruction (including CALL and RETURN) decoding can be done inearly decoding. The unconditional branch instruction should be the lastinstruction in the line which should be compared to the branchinformation in the ICFPC. If the unconditional branch instruction wasnot recorded in the branch global shift register, then the ICFPCre-fetches from the target of the unconditional branch and corrects thebranch tags in the branch global shift register. The CALL and RETURNinstructions are necessary for setting the return stack. The operandsand flags are sent directly to the register file and FIROB for accessingthe data. The operation of this block takes 2.5 clock cycles whichincludes prioritizing, aligning, decoding, and muxing of instructions todecode units.

Alignment of Instructions to Dispatched Positions

Instructions are sent as 8-byte blocks from the ICPDAT to the ICALIGN,and they are prioritized to 4 dispatched positions for early decodingand muxing to the 4 decode units. FIG. 20 of the 4 pipeline stages isshown for reference. Timing of fetching instructions to the decode unitsis as followed:

Cycle 1--Fetch from the array, the ICPDAT starts the scanning, and theICTAGV starts the tag comparison. IB(127:0) is muxed to IB1(191:0) foraligning to 4 dispatched positions.

Cycle 2--Complete the scanning of each individual 8-byte and align 7bytes for each instruction to the early decoding.

Cycle 3--Multiplex the instructions to 4 early-decoding units (thescanning logic can send up to 8 instructions) and decode up to 5prefixes and opcode for 2-dispatch positions instruction, and decode forthe register operands, flags, CALL/RETURN instructions.

Cycle 4--Multiplex the instructions to 4 decoding units, the prefix isthe combined prefix and the instructions from IB2(191:0). The decodeunit start the operand and flag decoding.

The operation in the scanning cycle:

If the scanning logic can dispatch all of first and second 8-bytes, itfetches another 16-bytes from the array.

If the first 8-bytes cannot be dispatched, the dispatched instructionsof the second 8-bytes in the ICALIGN are invalidated. The pre-decodebits of the first 8-byte is updated. The reading from the array isvoided.

If the first 8-bytes can be dispatched, and the second 8-bytes cannot bedispatched, then the pre-decode bits of the second 8-bytes are updated.The second 8-bytes are shifted to the first 8-bytes, and a new 8-bytesare fetched into the second 8-bytes.

Signals from the next stages can indicate the number of 8-bytes to send.

Each 8-byte block consists of the following information:

Number of valid instructions.

The valid bits for each instruction, the valid bits can continue to thenext 8-byte blocks.

The functional bits for each instruction, the functional bits cancontinue to the next 8-byte blocks.

The operation in prioritizing and aligning:

Four instructions are selected from IB1(191:0) to send to earlydecoding. Each instructions can have up to 7 bytes for decoding, 4prefix bytes, 1 opcode, 1 MODRM, and 1 SIB.

In the 8-byte block where the third instruction is started, this 8-byteblock should be kept until the next clock cycle. The appropriate HOLDsignals are generated to ICPDAT for accepting the next 8-byte blocks. Ifthe first two instructions require 2 dispatched positions each, then thethird instruction must wait until the next clock cycle. The decodeinformation is not available until the fourth ICLK. The thirdinstruction must remain in the IB1 latch until the first twoinstructions are decoded. Except for the SIB-byte instruction with earlyindication by the functional bit, the opcode must be decoded for fewother instructions. The information from decoding is too late to stopthe scanning cycle from sending the next 8-bytes. MROM instructionshould move to dispatch position 0 and wait for the micro-instructionsfrom MENG. MROM instruction remains in the first 8-byte block of IB2until completion of MROM instruction.

If the prefixes count for any instruction in the first 8-byte is greaterthan 4, ICALIGN can only accept another 8-byte. The detection logic isan AND gate of 6 valid functional bits.

Invalidate the dispatched instructions in ICALIGN.

FIG. 21 shows exemplary hardware in a block diagram format for aligningof instructions from Icache to Idecode.

Early Decoding

The early decoding functions of this block includes:

Combine all prefixes into one byte.

Detect a few special instructions and send into two dispatched positionsin the same line which will be dispatched to decode units.

Decode the unconditional jump instruction to generate taken branchaddress for the next fetch PC which should be from way and branchprediction. The CALL and RETURN are used to update the return stack.

Verify the predicted taken branch instruction.

Speculatively decode for register operands and flags, and generate theoperand size information.

Generate displacement size to read the displacement and immediate fieldsfrom the bus to the decode units.

A diagram showing exemplary hardware for early decoding of prefixes andopcodes is shown as FIG. 22.

The dispatched instructions from the ICPDAT are in two 8-byte blocks andarrange into three 8-byte blocks for aligning. The ICALIGN must selectthe first 4 instructions to mux the instruction bytes to early decodingunits. Each early decoding unit receives 7 bytes of instructions for thepossibility of 4 prefix bytes, 1 opcode byte, 1 MODRM byte, and 1 SIBbyte. Two conditions that can be detected early and fed back to theICPDAT for accepting the next blocks of instructions are: the SIB-byteinstruction, and the instruction with more than 4 prefixes. The otherconditions for instructions with 2 dispatched positions take longer todetect. The pre-decoded functional bits provide useful information. Inearly decoding, the valid bits for the instruction should be used toqualify the valid bytes of the instructions. Refer to a previous sectionfor encoding of the functional bit. For the first byte, indicated by thestart bit, if the functional bit is 0, the instruction is MROM, if thefunctional bit is 1, the instruction is fast path. The encoding for thefunctional bits of MROM instruction is inverted that of the fast pathinstruction. If the first functional bit is 0, all functional bits areinverted before qualifying with the valid bits for prefix and opcodedecoding. MROM instructions are handled in the MROM block. Detection ofMROM is passed through the early decode unit and hold formicro-instructions from MROM. In addition, the functional bits shouldalso be set for SIB-byte. The SIB-byte instruction takes two dispatchedpositions. ICALIGN needs to look at 2 bytes after the opcode byte forSIB-byte setting of the functional bit. The opcode byte can be detectedthe setting of the functional bits from the start bit. The logic can beas shown in FIG. 23.

Prefix Decoding

In early decode, 4 prefix decoding units are used for each prefix byte.All the prefixes are combined into a single encoded prefix for decodeunits. For each start bit, the ICALIGN logic looks at up to 7 functionalbits at a time. The start and end bits are used to validate and qualifythe functional bits for shifting and decoding of the instruction bytes.Decoding for the prefixes are as followed:

    ______________________________________                                        Prefix Encoded Prefix                                                         ______________________________________                                               0000 0000  No prefix                                                   0x0F   1xxx xxxx  2-byte escape                                               0x66   x1xx xxxx  operand size override, 16 or 32-bit data                    0x67   xx1x xxxx  address size override, 16 or 32-bit addr                    0xF0   xxx1 xxxx  LOCK prefix                                                 0x2E   xxxx 1000  CS segment override                                         0x36   xxxx 1001  SS segment override                                         0x3E   xxxx 1010  DS segment override                                         0x26   xxxx 1100  ES segment override                                         0x64   xxxx 1101  FS segment override                                         0x65   xxxx 1110  GS segment override                                         ______________________________________                                    

Another function of early decoding is to detect instructions which havemore than 4 prefix bytes. The instruction will not be dispatched in thenext cycle. For more than 4 prefix bytes, an extra clock cycle isneeded, after decoding the first 4 prefixes the instruction is shiftedby 4 bytes and to decode the prefixes and opcode again. The extraprefixes will override the previous encoding of the same prefix. Thepercentage of 5 or more prefixes is less than 0.01% for one bench markand less than 0.00% in all other bench marks.

Some notes on the prefix assumptions:

2-byte escape must be before the opcode. Multiple 2-byte escape prefixescause illegal opcode exception. Segment override changes the defaultsegment used for an instruction. Last prefix is used when multiplesegment prefixes are present. Instruction that does not reference asegment ignores segment prefixes.

Operand size toggles the operand size between 16 and 32. When theoperand size is 8, the operand size prefix is ignored. Multiple operandsize prefixes do the same as single operand size prefix.

Address size toggles the address size between 16 and 32. Stack access ofsome instructions and instruction without address calculation are notaffected by address size prefix. Multiple address size prefixes do thesame as single address size prefix.

A lock prefix is only allowed on certain instructions. When notapplicable, lock prefixes cause an illegal opcode exception. Someinstructions (i.e. EXCH) cause locked accesses by default without thelock prefix. Multiple lock prefixes do the same as single lock prefix.

Repeat for string instructions, repeat prefixes are ignored when notapplicable.

Double-Dispatch Decoding

Besides the SIB instruction which requires two dispatch positions, fewother instructions must go through decoding. The same opcode are sent totwo decode units with an encoded field for indication of the first orsecond dispatch position. The list of 2-dispatch position instructionsare: PUSH, POP, RETURN, CALL, MUL, IMUL, LOOP, JCXZ, JECXZ, and LEAVE.The above instructions have either two destination registers, or twoload/store operations, or more than two input operands (not counting theimmediate and displacement). The above instructions with SIB should goto MROM. PUSH from memory and POP to memory instructions can becomeregister if the MOD field is 11. In this case, the PUSH from memoryshould take only one dispatch position, and the POP to memory shouldtake 2 dispatch positions instead of going to MROM.

    ______________________________________                                        Opcode    2-dispatch positions                                                ______________________________________                                                  0000000000  Single instruction                                                0xxxxxxxxx  First rop of the 2-dispatch                                                   positions instruction                                             1xxxxxxxxx  Second rop of the 2-dispatch                                                  positions instruction                                   0xFF /6   x1xxxxxxxx  PUSH from memory                                        0x58      xx1xxxxxxx  POP into register                                       0x1F                  POP into DS                                             0x07                  POP into ES                                             0x17                  POP into SS                                             0xDF A1               POP into FS                                             0xDF A9               POP into GS                                             0xF7 /4   xxx1xxxxxx  MUL instruction - word/dword                            0xF7 /5   xxx1xxxxxx  IMUL instruction - word/dword                           0xC9      xxxx1xxxxx  LEAVE                                                   0xE0      xxxxx1xxxx  LOOP with ZF=0                                          0xE1                  LOOP with ZF=1                                          0xE2                  LOOP                                                    0xE3      xxxxxx1xxx  JCXZ & JECXZ                                            0xE8      xxxxxxx1xx  CALL near, displacement relative                        0xFF /2               CALL near, register indirect                            relative                                                                      0xC3      xxxxxxxx1x  RETURN near                                             0xC2                  RETURN near, immediate                                  FUNC bit set                                                                            xxxxxxxxx1  SIB-byte instruction                                    ______________________________________                                    

Handling of Branch Instructions

For unconditional branch instructions, the branch is always taken, andthe fetching should stall until the target address is known. There arethree types of unconditional branch instructions, CALL/INT, RETURN/IRET,and unconditional jump. These branch instructions should be predictedtaken. Processor 500 should implement a return stack, and as the CALLand INT instructions are in decode, the next instruction PC will bewritten into the return stack for future reference. The RETURN and IRETinstructions will get the target address from the return stack, it isnot necessary to keep the RETURN and IRET instructions in the ICNXTBLK.The target address calculation for non-conditional jump requires anadder to speculatively add the PC to displacement which is done in theICFPC. The important features of the return stack are discussed indetail in ICFPC.

    ______________________________________                                        JUMP        1110 10x1     PC = PC + imm                                       JUMP        FA            PC = CS:imm                                         JUMP        FF xx100xxx   PC = r/m32                                          JUMP        FF xx101xxx   PC = CS: m16:32!                                    CALL        E8            PC = PC + imm                                       CALL        FF xx010xxx   PC = r/m32                                          CALL        FF xx011xxx   PC = CS: m16:32!                                    CALL        98            PC = CS:imm                                         RETURN      C2, C3, CA, CB                                                                              PC =  return stack!                                 INT         CC, CD, CE    PC is from MENG                                     IRET        CF            PC =  return stack!                                 ______________________________________                                    

If the above instructions are not detected in the way and branchprediction, Icache should re-fetch from the new branch target. INTinstruction is not in the ICNXTBLK, detection of the INT will stalluntil the MENG or FIROB request a new PC. The CALL instruction has theoffset matched with the branch taken offset in the ICFPC. The RETURN andIRET are predicted by the way prediction. The ICFPC keeps status bitsfor each line of instruction to control the proper flow of instructions.

For conditional branch instructions, the ICNXTBLK is in total control ofthe prediction. Only the taken branch is recognized in the ICALIGN. Onlyone taken branch is possible per instruction line. The ICALIGN notifiesthe ICFPC to send the PC's for each line dispatched to the decode units.Only one taken branch is allowed per 4 instructions sent to the decodeunits. The branch target address and the branch tag from the globalshift register should be routed along with any branch instruction(include unconditional jump) to the FIROB in case of mis-prediction.

The ICFPC always sends the current PC to FIROB and decode units. Ifthere is an indication of the dispatching of instruction from a new8-byte block, the next PC is sent. If the instruction of a new 8-byteblock is to decode unit 0, then the next PC is sent as first PC, elsethe next PC is sent as second PC. The PC is 32-bits which indicates thefull address of the instruction. The offset of the instruction is forthe next instruction.

MODRM and SIB Decoding

With up to four instructions dispatched per clock cycle, eight possibleread operands must be checked for data dependency. The FIROB checks fordependency against all previously dispatched instructions (up to 5 linesor 20 instructions) and within the current line. In the FIROB, thenumber of comparators are 20 by 8 for the 6-bit register operands. It isimportant to decode the register operands as soon as possible. Flags arein the same category with the operands which need early indication. Someof the X86 opcodes have implied references to registers and flags. Theregister operands are from MODRM byte. In early decoding, the registersare speculatively decoded and sent to dependency checking. In decodeunits, the direction and validation of the registers are decoded to setthe statuses in the FIROB and control the issuing of operands tofunctional units.

The MODRM has 3 fields: REG, MOD, and R/M. The REG field is sent to thefirst operand and the R/M field is sent to the second operand. The R/Mis not a valid register operand for the following MOD field:

    ______________________________________                                        16-bit addressing                                                                          MOD = 00 and R/M = 110,                                                                        16-bit                                                                        displacement                                    32-bit addressing                                                                          MOD = 00 and R/M = 100,                                                                        SIB                                                          MOD = 00 and R/M = 101,                                                                        32-bit                                                                        displacement                                                 MOD = 01 and R/M = 100,                                                                        SIB, 8-bit disp                                              MOD = 10 and R/M = 100,                                                                        SIB, 32-bit                                                                   displacement                                    ______________________________________                                    

For 32-bit address mode, the SIB byte is specified as above. Theinstruction is dispatched in two issue positions. The first Rop is anadd of the Base to Index with indication to the FIROB of invalid result.The second Rop is the actual instruction without the SIB byte, thesecond Rop generates a dependency tag related to the first Rop. For the16-bit addressing mode, the MODRM also indicates the base and indexwhich are equivalent to the SIB byte for 32-bit addressing mode. Thecoding of MODRM is as follows:

    ______________________________________                                        MOD = 0x, 10                                                                              R/M = 000 Base = BX Index = SI                                    MOD = 0x, 10                                                                              R/M = 001 Base = BX Index = DI                                    MOD = 0x, 10                                                                              R/M = 010 Base = BP Index = SI                                    MOD = 0x, 10                                                                              R/M = 011 Base = BP Index = DI                                    ______________________________________                                    

For MOD=11, the R/M field specifies the register based on the widthfield and the data sizes. The width field is the least significant bitof the opcode, if w=0, the data size is 8 bit, if w=1, the data size is16 or 32 bits. The data size is from Code Segment register with possibleoverride from prefix.

    ______________________________________                                        MOD       R/M    8-bit        16-bit                                                                             32-bit                                     ______________________________________                                        11        000    AL           AX   EAX                                        11        001    CL           CX   ECX                                        11        010    DL           DX   EDX                                        11        011    BL           BX   EBX                                        11        100    AH           SP   ESP                                        11        101    CH           BP   EBP                                        11        110    DH           SI   ESI                                        11        111    BH           DI   EDI                                        ______________________________________                                    

The displacement field is also decoded from the MODRM:

    ______________________________________                                                         displacement size                                                                           displacement size                              MOD    R/M       16-bit addressing                                                                           32-bit addressing                              ______________________________________                                        00     110       16-bit        no displacement                                00     101       no displacement                                                                             32-bit                                         01     xxx       8-bit         8-bit                                          10     xxx       16-bit        32-bit                                         ______________________________________                                    

Opcode Decoding for Operands and Flags

Besides register addresses from the MODRM byte the instructions withimplied register in the opcode should also be decoded:

    ______________________________________                                        PUSH     0101 0nnn A, C, D, B, SP, BP, SI, DI                                                                   R                                           POP      0101 1nnn A, C, D, B, SP, BP, SI, DI                                                                   W                                           LEAVE    1100 1001 EBP, ESP       RW                                          ALU OP   00xx x100 AL             RW                                                   1000 0000                                                            ALU OP   00xx x101 AX, EAX        RW                                                   1000 00x1                                                            MUL -byte                                                                              1111 0110 AX             W                                           SHIFTD   OF 1010 x101 CL          R                                           ROT/SHF  1101 001x CL             R                                           INC      0100 0nnn A, C, D, B, SP, BP, SI, DI                                                                   RW                                          DEC      0100 1nnn A, C, D, B, SP, BP, SI, DI                                                                   RW                                          BSWAP    0F 1100 1nnn A, C, D, B, SP, BP, SI, DI                                                                RW                                          CBW      1001 1000 A              RW                                          SAHF     1001 1110 AH             W                                           LAHF     1001 1111 AH             R                                           MOVE     1010 000x A              W                                           MOVE     1010 001x A              R                                           MOVE     1011 0nnn AL, CL, DL, BL, AH, CH, DH, BH                                                               W                                           MOVE     1011 1nnn A, C, D, B, SP, BP, SI, DI                                                                   W                                           ______________________________________                                    

The MUL and IMUL instructions should have the first issue position withEAX destination and the second issue position with EDX destination. Thesecond Rop is the actual instruction since the first Rop can be the addfor index and base of the SIB-byte. For the stack operations,POP/PUSH/CALL/RETURN instructions, the ESP must be read and written. Theinstruction is an add of ESP to the address size on the immediate busfor the functional unit. The POP and RETURN instruction read the ESP inthe first Rop and add to the ESP in the second ROP. The RETURN withimmediate for adding to the ESP must be handled as two operations in thefunctional units. Opcode decoding is needed to generate the ESP pointersfor the instructions:

    ______________________________________                                        0x8F        POP into memory                                                   0x58        POP into register                                                 0x1F        POP into DS                                                       0x07        POP into ES                                                       0x17        POP into SS                                                       0x0F A1     POP into FS                                                       0x0F A9     POP into GS                                                       0x61        POP into general registers                                        0x9D        POP into EFLAGS                                                   0xFF        PUSH from memory                                                  0x50        PUSH from register                                                0x6A        PUSH from immediate byte                                          0x68        PUSH from immediate word/dword                                    0x0E        PUSH from CS                                                      0x16        PUSH from SS                                                      0x1E        PUSH from DS                                                      0x06        PUSH from ES                                                      0x0F A0     PUSH from FS                                                      0x0F A8     PUSH from GS                                                      0x60        PUSH from general registers                                       0x9C        PUSH from EFLAGS                                                  0xE8        CALL near, displacement                                           0xFF /2     CALL near, register/memory indirect                               0xC3        RETURN near, ESP = ESP + 2/4                                      0xC2 /2     RETURN near, ESP = ESP + 2/4 + imm16                              0xC9        LEAVE, ESP = EBP + 2/4, EBP =  EBP!                               ______________________________________                                    

A constant field is generated for the ESP calculation. Depending on theaddress size, a constant of 2 or 4 is needed for the above instructions.For JCXZ and JECXZ instructions, a constant of 0 is needed for the zerodetection. For LOOP instructions, a constant of 1 is needed fordecrementing of the counter.

The decoding of the flags should be done in early decoding. The statusflag information is from the appendix on FLAG CROSS-REFERENCE in the X86manual. The instructions must provide the reading and writing of thestatus flags in the same manner as the operands. The flags are set up inthree groups:

CF-carry flag.

OF-overflow, SF-sig, ZF-zero, PF-parity, AF-auxiliary carry, andDF-direction flags.

The rest of the control flags in the EFLAG register.

The first two flag groups have dependency checking in the FIROB andforwarding in the reservation station. The second group is implementedwith three sub-groups, OF, DF, and {SF, ZF, AF, and PF} (4F). Thedependency checking of the second group is similar to that of theoperand, if the dependency is wide-to-narrow then it is okay forforward, if the dependency is narrow-to-wide then the instruction stallsin decode units until the dependency is removed. The last flag groupwill stall the instruction in decode units if there is any dependency.The decoding of the status flags is as followed:

    ______________________________________                                        Instruction                                                                              opcode       read flags write flags                                ______________________________________                                        ADC, SBB   0001 x0xx    CF         CF, OF, 4F                                            0001 xx0x                                                                     8 000x xx01xxxx                                                               8 0011 xx01xxxx                                                    ALU OP     00xx x100               CF, OF, 4F                                 SHIFTD     0F 1010 x101            CF, OF, 4F                                 RCL/RCR    D 00xx xx01xxxx                                                                            CF         CF, OF                                                C 000x xx01xxxx                                                    ROL/ROR    D 00xx xx00xxxx         CF, OF                                                C 000x xx00xxxx                                                    SHF        D 00xx xx10xxxx         CF, OF, 4F                                            D 00xx xx111xxx                                                               C 000x xx10xxxx                                                               C 000x xx111xxx                                                    INC        0100 0xxx               CF, OF, 4F                                 DEC        0100 1xxx    CF, OF, 4F                                            LAHF       1001 1111               CF, OF, 4F                                 SAHF       1001 1110               CF, OF, 4F                                 CLC, SETC  1111 100x               CF                                         CMC        1111 0101    CF         CF                                         CLD, SETD  1111 110x               DF                                         CLI, SETI  1111 101x               IF                                         SET        0F 1001 001x CF                                                               0F 1001 011x CF, ZF                                                           0F 1001 000x OF                                                               0F 1001 11xx OF, 4F                                                           0F 1001 010x 4F                                                               0F 1001 10xx 4F                                                    JCCB       0111 001x    CF                                                               0111 011x    CF, ZF                                                           0111 000x    OF                                                               0111 11xx    OF, 4F                                                           0111 010x    4F                                                               0111 10xx    4F                                                    JCCW       0F 1000 001x CF                                                               0F 1000 011x CF, ZF                                                           0F 1000 000x OF                                                               0F 1000 11xx OF, 4F                                                           0F 1000 010x 4F                                                               0F 1000 10xx 4F                                                    BIT        0F 1010 x011            CF                                                    0F 1011 x011                                                                  0F 1011 101x                                                       ______________________________________                                    

MROM Register Operand Decoding

To not effect the MODRM decoding for Processor 500 fast path, themicrocode uses the prefix for extending the register field. The extendedregister field for microcode will be concatenated with the MODRMregister field to address the full 64 register file. For Processor 500fast path instruction, the extended register field is forced to zero.The MODRM, the displacement, and the immediate field can be read fromthe global decoding registers. A diagram of the MROM register decodingis shown as FIG. 24.

Multiplexing of Instruction to Decode Units

The format of instructions dispatched to the decode unit includes oneencoded prefix byte, which may not be valid, speculative registeroperands and flags, 2-dispatch position control bits, 11-bit opcode,3-bit constant, up to 4 bytes of displacement, and 4 bytes of immediatefield. During early decode, the displacement size and field isdetermined and read directly from the IB2(191:0) to the decode units.The immediate field is speculative of 4 bytes after the displacementfield. Two-dispatch position instructions cause shifting of the nextinstructions to the next issue positions. If the instruction at 0 takes2 dispatch positions, then only instruction at 1 is allowed to take 2dispatch positions. The controls for reading of the displacement andimmediate fields are shifted according to the 2-dispatch positioninstruction. A diagram showing the multiplexing of 2-dispatch positioninstructions to the decode units is shown as FIG. 25.

The first four instructions from scanning can go through early decoding.If a 2-dispatch position instruction is detected, the followedinstruction must be shifted to the next decode unit or wait until thenext cycle. If the 2-dispatch position instruction is at the last decodeunit, it also must wait until the next cycle.

MENG should provide the same format as with fast-path instructions.Detection of MROM instruction should delay until the next cycle and indispatch position 0. The MROM instruction should locate at the first8-byte block on the IB2(191:0). MENG can provide the pointers to readthe displacement and immediate fields to decode units directly fromIB2(191:0). The MROM instruction is shifted out of the dispatch position0 along with the last line of the micro-code sequence, the next set ofinstructions will assume normal dispatching.

Exception Instructions

The illegal instruction is detected in ICPRED which does not set thevalid bit in the ICTAGV. The illegal instruction must be dispatched todecode unit 0 by itself with ICERROR. Similarly for TLB misses, externalbus error, and greater-than-15-byte instruction, the error conditionsare passed to decode unit 0. The limit checking of each instruction isdone in the ICFPC, the ICFPC will notify the ICALIGN with the limitviolated byte position. The illegal instruction is higher priority thanlimit violation.

Signal List

STARTPTR(3:0)--Input from ICFPC indicates the byte position of thepre-decoded bytes for current instruction. The start byte should be setat this pointer.

HLDISP(1:0)--Output to ICFPC indicates the first (bit 0) and/or thesecond (bit 1) 8-byte of the current line has been dispatched to decodeunits.

NEWLINE(1:0)--Input from ICFPC indicates the first (bit 0) or the second(bit 1) 8-byte is the starting of a new line. The byte positions ofinstructions sending to the decode units should be adjusted accordingly.

IC0VAL1(7:0)--Input from ICDPAT indicates the valid bytes forinstruction 0 of the first 8-byte.

IC1VAL1(7:0)--Input from ICDPAT indicates the valid bytes forinstruction 1 of the first 8-byte.

IC2VAL1(7:0)--Input from ICDPAT indicates the valid bytes forinstruction 2 of the first 8-byte.

IC3VAL1(7:0)--Input from ICDPAT indicates the valid bytes forinstruction 3 of the first 8-byte.

IC0VAL2(7:0)--Input from ICDPAT indicates the valid bytes forinstruction 0 of the second 8-byte.

IC1VAL2(7:0)--Input from ICDPAT indicates the valid bytes forinstruction 1 of the second 8-byte.

IC2VAL2(7:0)--Input from ICDPAT indicates the valid bytes forinstruction 2 of the second 8-byte.

IC3VAL2(7:0)--Input from ICDPAT indicates the valid bytes forinstruction 3 of the second 8-byte.

IC8BSEL(5:0)--Output to ICALIGN controls the muxes from IB(127:0) toIB1(191:0). Bit 5:3 is for the upper 8-byte IB(127:64) and bit 2:0 isfor the lower 8-byte IB(63:0). 001 is for muxing to the first 8-byteIB1(63:0), 010 is for muxing to the second 8-byte, and 100 is for muxingto the third 8-byte.

NSHIFT1(4:0)--Output to ICALIGN indicates that for the first 8-byteblock, the look-ahead shifting of the next 8-byte instructions to decodeunits. A shift by 4 means that no instruction from the next eight byteis sent to the decode unit.

NSHIFT2(4:0)--Output to ICALIGN indicates that for the second 8-byteblock, the look-ahead shifting of the next 8-byte instructions to decodeunits. A shift by 4 means that no instruction from the next eight byteis sent to the decode unit.

ENDINST--Input from ICPRED indicates that pre-decoding is completed forthe current instruction.

MROMEND--Input from MENG indicates completion of the MROM.

PDMROM--Input from ICPRED indicates the current instruction is MROM.

PD8BSEL(5:0)--Output to ICALIGN controls the muxes from IB(127:0) toIB1(191:0). Bit 5:3 is for the upper 8-byte IB(127:64) and bit 2:0 isfor the lower 8-byte IB(63:0). 001 is for muxing to the first 8-byteIB1(63:0), 010 is for muxing to the second 8-byte, and 100 is for muxingto the third 8-byte.

PFUNCB(2:0)--Input from ICPRED indicates the functional bits for currentinstruction. Only three bytes (opcode, MODRM, and SIB) are needed forearly decode of the instruction. The prefixes are encoded into onesingle byte.

PDVAL(10:0) Input from ICPRED indicates the valid bytes for pre-decodeinstruction to decode unit 0. This signal is to validate the bytes indecode. The maximum length of the instruction is 11 bytes.

PDSHF(5:0)--Input from ICPRED indicates which bytes to shifting to earlydecoding. The ICPRED must put the current line into the IB1(191:0) forshifting. This is necessary because the long instruction can span morethan three 8-byte blocks. The upper 2 bits to select which 8-byte block,the ICALIGN can accumulate the pre-decode instructions while waiting forthe MROM instructions.

PDPREF(7:0)--Input from ICPRED indicates the encoded prefix byte. Thisis to avoid the pre-decode instruction taking 2 cycles in early decodingbecause of more than 4 prefixes.

PDTAKEN--Input from ICPRED indicates the current instruction is takenbranch (the taken branch target can be calculated.

NEXTB(1:0)--Output to ICPDAT indicates that ICALIGN can accept any8-byte block. This signal includes other hold inputs from MROM andIdecode.

DOUSEFL (4:0)

DOWRFL(4:0)--Output to FIROB indicates the type of flag uses/writes forthis instruction of decode unit 0:

xxxx1 CF-carry flag,

xxx1x DF-direction flag,

xx1xx OF-overflow flag,

x1xxx SF-sign, ZF-zero, PF-parity, and AF-auxiliary carry 1xxxx controland processor flags,

D1USEFL(4:0)

D1WRFL(4:0)--Output to FIROB indicates the type of flag used/written forthis instruction of decode unit 1.

D2USEFL(4:0)

D2WRFL(4:0)--Output to FIROB indicates the type of flag used/written forthis instruction of decode unit 2.

D3USEFL(4:0)

D3WRFL(4:0)--Output to FIROB indicates the type of flag used/written forthis instruction of decode unit 3.

RD0PTR1(5:0)--Indicates the register address for operand 1 of decodeunit 0. The MROM is responsible to send bit 5:3 for the MROM register.

RD1PTR1(5:0)--Indicates the register address for operand 1 of decodeunit 1. The MROM is responsible to send bit 5:3 for the MROM register.

RD2PTR1(5:0)--Indicates the register address for operand 1 of decodeunit 2. The MROM is responsible to send bit 5:3 for the MROM register.

RD3PTR1(5:0)--Indicates the register address for operand 1 of decodeunit 3. The MROM is responsible to send bit 5:3 for the MROM register.

RD0PTR2(5:0)--Indicates register address for operand 2 of decode unit 0.The MROM is responsible to send bit 5:3 for the MROM register.RD1PTR2(5:0)--Indicates register address for operand 2 of decode unit 1.The MROM is responsible to send bit 5:3 for the MROM register.

RD2PTR2(5:0)--Indicates register address for operand 2 of decode unit 2.The MROM is responsible to send bit 5:3 for the MROM register.

RD3PTR2(5:0)--Indicates register address for operand 2 of decode unit 3.The MROM is responsible to send bit 5:3 for the MROM register.

IDxDAT(1:0)--Output to indicates the data size information. 01-byte,10-half word, 11-word, 00-not use.

ICOPOS1(3:0)--Input from ICDPAT indicates the PC's byte position forinstruction 0 of the first 8-byte.

IC1POS1(3:0)--Input from ICDPAT indicates the PC's byte position forinstruction 1 of the first 8-byte.

IC2POS1(3:0)--Input from ICDPAT indicates the PC's byte position forinstruction 2 of the first 8-byte.

IC3POS1(3:0)--Input from ICDPAT indicates the PC's byte position forinstruction 3 of the first 8-byte.

IC0POS2(3:0)--Input from ICDPAT indicates the PC's byte position forinstruction 0 of the second 8-byte.

IC1POS2(3:0)--Input from ICDPAT indicates the PC's byte position forinstruction 1 of the second 8-byte.

IC2POS2(3:0)--Input from ICDPAT indicates the PC's byte position forinstruction 2 of the second 8-byte.

IC3POS2(3:0)--Input from ICDPAT indicates the PC's byte position forinstruction 3 of the second 8-byte.

ICPOSx(4:0)--Output to Idecode indicates the PC's byte position of thenext instruction. ICALIGN uses the above signal to generate the offsetof the next instruction which will be used for branch mis-prediction.The most significant bit indicates the next instruction started at thenext line.

ICBRN(3:0)--Input from ICNXTBLK indicates the branch taken predictionthe lines of instructions being fetched.

ICBTAG1(3:0)--Output to Idecode indicates the position of the firsttarget branch instruction with respect to the global shift register incase of branch mis-prediction. The branch can be taken or non-taken,branch tag must be sent with all branch instruction.

ICBTAG2(3:0)--Output to Idecode indicates the position of the secondtarget branch instruction with respect to the global shift register incase of branch mis-prediction. The branch can be taken or non-taken,branch tag must be sent with all branch instruction.

RETPRED--Input from ICNXTBLK indicates the current prediction of thereturn instruction of the fetched line. The return instruction must bedetected in the current line of instruction or the Icache must bere-fetched from a new line.

UNJMP(3:0)--Output to ICCNTL indicates the unconditional branchinstruction needs to calculate target address.

BRNTKN(3:0)--Output indicates which decode unit has a predicted takenbranch. The operand steering uses this signal to latch and sendBTADDR(31:0) to the functional unit.

BRNINST(3:0)--Output indicates which decode unit has a global branchprediction. The operand steering uses this signal to latch and sendICBTAG1(3:0) and ICBTAG2(3:0) to the functional units.

CALLDEC(3:0)--Output to ICFPC indicates the CALL instruction isdetected, the return stack should be updated with the PC address ofinstruction after CALL. The line must be taken.

NORETB--Output to ICFPC indicates no RETURN instruction is detected forthe current line. This signal responses to RETPRED. The ICFPC should usethe branch prediction of BTAKEN to fetch the next line.

RETDEC(3:0)--Output to ICFPC indicates a RETURN instruction is detectedfor the way prediction to update if no RETURN was predicted. The ICFPCneeds to keep track of the branch instructions for 4 lines and restoresthe global shift register in case of mis-predicting the RETURNinstruction in Idecode.

ICnPREF(7:0)--Output to decode unit n indicates the encoded prefix byte.IC2ROPn(9:0)--Output to decode unit n indicates 2-dispatch positionsinstruction. Bit 9 indicates the first rop or second rop of the2-dispatch positions instruction, bits 8:0 indicate the type ofinstructions.

NODEST(3:0)--Output to FIROB indicates no destination for the first ropof the SIB-byte instruction.

DEPTAG(3:1)--Output to FIROB indicates forced dependency tag on thefirst instruction; the second rop of the SIB-byte instruction.

DISPBRN1(1:0)--Output to ICFPC indicates the first predicted branchinstruction dispatched to decode unit. This signal is the OR ofICxBRNB1. Bit 0 is for the first 8-byte and bit 1 is for the second8-byte of the line if NEWLINE(1) is set for the second 8-byte.

DISPBRN2(1:0)--Output to ICFPC indicates the second predicted branchinstruction dispatched to decode unit.

This signal is the OR of ICxBRNB2. Bit 0 is for the first 8-byte and bit1 is for the second 8-byte of the line if NEWLINE(1) is set for thesecond 8-byte.

BVALL--Input from ICNXTBLK to qualify BTAG1, indicates the first branchtarget is greater or equal to STARTPTR.

BVAL2--Input from ICNXTBLK to qualify BTAG2, indicates the second branchtarget is greater or equal to STARTPTR.

REFRESH2--Input from Idecode indicates current line of instructions willbe refreshed and not accept new instructions from Icache.

IB(127:0)--Input from ICSTORE indicates the new line of instructions tobe sent to decode units.

IB1(191:0)--Internal indicates the combined instruction line foraligning from the array.

IB2(191:0)--Output indicates the combined instruction line fordispatching to decode units, the displacement and immediate fields areread from this bus.

MROMEN--Input from MENG indicates the micro-instructions is sent toIdecode instead of the Icache.

M0USEFL(4:0)

M0WRFL(4:0)--Input from MENG indicates the type of flag used/written forthis micro-instruction of decode unit 0:

xx1 CF-carry flag,

x1x OF-overflow flag,

1xx SF-sign, ZF-zero, PF-parity, and AF-auxiliary carry

M1USEFL(4:0)

M1WRFL(4:0)--Input from MENG indicates the type of flag used/written forthis micro-instruction of decode unit 1.

M2USEFL (4:0)

M2WRFL(4:0)--Input from MENG indicates the type of flag used/written forthis micro-instruction of decode unit 2.

M3USEFL(4:0) M3WRFL(4:0)--Input from MENG indicates the type of flagused/written for this micro-instruction of decode unit 3.

MINS0(63:0)--Input from MENG indicates the displacement and immediatefield of micro-instruction sending to decode 0.

MINS1(63:0)--Input from MENG indicates the displacement and immediatefield of micro-instruction sending to decode 1.

MINS2(63:0)--Input from MENG indicates the displacement and immediatefield of micro-instruction sending to decode 2.

MINS3(63:0)--Input from MENG indicates the displacement and immediatefield of micro-instruction sending to decode 3.

MR0OPC(7:0)--Input from MENG to decode unit 0 indicates the opcode byte.

MR1OPC(7:0)--Input from MENG to decode unit 1 indicates the opcode byte.

MR2OPC(7:0)--Input from MENG to decode unit 2 indicates the opcode byte.

MR30PC(7:0)--Input from MENG to decode unit 3 indicates the opcode byte.

MR0EOP(2:0)--Input from MENG to decode unit 0 indicates the extendedopcode field.

MR1EOP(2:0)--Input from MENG to decode unit 1 indicates the extendedopcode field.

MR2EOP(2:0)--Input from MENG to decode unit 2 indicates the extendedopcode field.

MR3EOP(2:0)--Input from MENG to decode unit 3 indicates the extendedopcode field.

MR0SS(1:0)--Input from MENG to decode unit 0 indicates the scale factorof the SIB byte.

MR1SS(1:0)--Input from MENG to decode unit 1 indicates the scale factorof the SIB byte.

MR2SS(1:0)--Input from MENG to decode unit 2 indicates the scale factorof the SIB byte.

MR3SS(1:0)--Input from MENG to decode unit 3 indicates the scale factorof the SIB byte.

MBYTEPTR(3:0)--Output to MENG indicates the byte pointer for the currentMROM instruction. MROM uses the pointer to read the instruction onIB(127:0), ICEND(15:0), and ICFUNC(15:0).

ICMROM--Output to MENG indicates the current instruction is MROM. TheMROM instruction may take two cycles to read the IB, ICEND, and ICFUNC.

ENDINST--Input from ICPRED indicates that pre-decoding is completed forthe current instruction. The byte position of the branch instruction isfrom STARTPTR. The selected instruction from IB should be sent to decodeunit 0.

PDJMP--Input from ICPRED indicates the current instruction is a branchinstruction. The ICNXTBLK sends BTAG1 and the ICALIGN sends IC0BRNBN1(0)to the decode unit.

ICVALI(3:0)--Output to Idecode indicates valid instructions. NOOP isgenerated for invalid instruction.

IC0OPC(7:0)--Output to decode unit 0 indicates the opcode byte.

IC1OPC(7:0)--Output to decode unit 1 indicates the opcode byte.

IC2OPC(7:0)--Output to decode unit 2 indicates the opcode byte.

IC3OPC(7:0)--Output to decode unit 3 indicates the opcode byte.

IC0EOP(2:0)--Output to decode unit 0 indicates the extended opcodefield.

IC1EOP(2:0)--Output to decode unit 1 indicates the extended opcodefield.

IC2EOP(2:0)--Output to decode unit 2 indicates the extended opcodefield.

IC3EOP(2:0)--Output to decode unit 3 indicates the extended opcodefield.

IC0SS(1:0)--Output to decode unit 0 indicates the scale factor of theSIB byte.

IC1SS(1:0)--Output to decode unit 1 indicates the scale factor of theSIB byte.

IC2SS(1:0)--Output to decode unit 2 indicates the scale factor of theSIB byte.

IC3SS(1:0)--Output to decode unit 3 indicates the scale factor of theSIB byte.

DISPTR0(6:0)--Output to decode unit 0 indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, bit 6:5 is thesize, and bit 4:3 indicates which 8-byte block. Bit 6:5=00 indicates nodisplacement.

DISPTR1(6:0)--Output to decode unit 1 indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, bit 6:5 is thesize, and bit 4:3 indicates which 8-byte block. Bit 6:5=00 indicates nodisplacement.

DISPTR2(6:0)--Output to decode unit 2 indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, bit 6:5 is thesize, and bit 4:3 indicates which 8-byte block. Bit 6:5=00 indicates nodisplacement.

DISPTR3(6:0)--Output to decode unit 3 indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, bit 6:5 is thesize, and bit 4:3 indicates which 8-byte block. Bit 6:5=00 indicates nodisplacement.

IMMPTR0(4:0)--Output to decode unit 0 indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, and bit 4:3indicates which 8-byte block. Decoding of all opcodes is needed todetect immediate field.

IMMPTR1(4:0)--Output to decode unit 1 indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, and bit 4:3indicates which 8-byte block. Decoding of all opcodes is needed todetect immediate field.

IMMPTR2(4:0)--Output to decode unit 2 indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, and bit 4:3indicates which 8-byte block. Decoding of all opcodes is needed todetect immediate field.

IMMPTR3(4:0)--Output to decode unit 3 indicates the displacement pointerand size. Bits 2:0 is the pointer to the 8-byte block, and bit 4:3indicates which 8-byte block. Decoding of all opcodes is needed todetect immediate field.

CONSTn(2:0)--Output to decode unit n indicates the constant foradd/substract to ESP of the two-dispatch position instruction.

ICCNTL

The ICCNTL is the main control in the ICACHE. The state machine controlsreading and writing of the arrays in the first two pipeline stages. Thenext two pipeline stages are controlled by the ICALIGN. The detection ofthe unconditional branch instruction are treated as mis-prediction tofetching a new instruction stream without updating the ICNXTBLK. Itlatches the inputs at the beginning of ICLK and generates controlsignals to the arrays for the next cycle by the end of ICLK. There are afew signals from the arrays, such as TAGHIT, to ICCNTL in early ICLKinstead of previous phase. The state machine can be jammed into acertain state with late arriving signals from branch mis-prediction andbranch prediction. The IRESET jams the state machine to Idle state,initializes the code segment register, and clears all the status ofICACHE. For external fetch on Processor 500, there will be dedicatedbuses for address to MMU and instructions from the pads, the statemachine for external fetch is not needed. The preliminary state machinedefinition and transitions are from the definition of Processor 500 andis shown as FIG. 26.

The ICCNTL provides the signals to read and write the cache arrays. Forsimple operation, the array has the same inputs as the ICCNTL to avoidthe speed path through the state machine.

The Move-To/From-Special-Register instruction can occur at any time andsteal one cycle at a convenient time away from the normal operation. Themove to/from instruction is for testing purposes and happens withinstruction cache disabled. The external fetch can be concurrent withSpecial Register access. The state machine implements with latches tohold the input and output conditions during the Special Register access.The operation of Special Register takes 5 cycles and is as followed:

Command and tag on the SRBB is received with indication by SRB₋₋ VAL.The tag on SRBB is decoded to recognized the access is for Icache.

The address of the array is on SRBB. This address should be latched intothe ICFPC.

For reading, the array is read in this cycle. For writing, the data islatched from SRBB.

The data is driven on ICSRBB.

For reading, the data is forward from ICSRBB to SRBB. For writing, thedata is written into the array. ICTAR₋₋ VAL is sent to SRB in thiscycle.

STATE0: Idle State

The Idle state is forced by IRESET, Branch Mis-prediction, or EXCEPTION,and waits for taken branch target. This is a default state. If the stateis forced by branch mis-prediction, it provides Icache control signalsto write the Branch Holding Register into the ICNXTBLK. This statetransfers to the Cache Access state when the taken branch address isvalid, the transfer provides all Icache control signals for reading thearray.

STATE1: Cache Access State

The Icache is being accessed. The TAGHIT is not know until next clockcycle; the assumption is HIT and accessing the next block, the nextblock address can either be from the ICNXTBLK or sequential. This stateprovides Icache control signals for reading the array. When the TAGHITis known, if there is no holding due to breaking up of the instructionline or invalid pre-decode data, then the state remains in Cache Accessstate, else the state will transition to Cache Hold state. Thetransition to Cache Hold state will need to provide Icache controlsignals for reading the next block of the array. If miss in the Icache,the state is transferred to Cache Miss state. The miss can either be thetag or LV miss. The transfer to Cache Miss state must provides Icachecontrol signals to write the Branch Holding Register into the ICNXTBLK.

STATE2: Cache Hold State

The state waits for the whole line of instruction to be sent to decodeunits. Icache control signals for reading of next block is continuouslyprovided. As soon as the ICALIGN block can accept the next line, thestate transfers to the Cache Access state.

STATE3: Cache Miss State

The Cache Miss state makes a request to the CMASTER and waits for aresponse. There are two different responses, the first response is thenew mapping of the PC (the instructions, pre-decode data, and branchprediction are still valid), the second response is fetched instructionsfrom external memory. The new mapping of the PC includes setting of theLV bit and writing of new SU and tag. For the first case, the state istransferred to Recovery state, the Icache control signals are to writethe ICTAGV and read the ICSTORE, ICPDAT, and ICNXTBLK. For the secondcase, the state is transferred to the Pre-fetch state, and the Icachecontrol signals are to write the ICTAGV and ICSTORE.

STATE4: Recovery State

The Recovery state is a temporary state before transfers to the CacheAccess State. This state provides Icache control signals for reading thearray.

STATE5: Pre-fetch State

This state sends the instruction from the pre-fetch buffer to the ICPREDfor pre-decoding. The pre-fetch buffer accepts instructions until full,and handshakes with the BIU to stop fetching so as not to overfill thebuffer. As the current line is written into the ICSTORE array, thepre-fetch buffer can shift in a new line. The writing of the new linewaits for the completion of pre-decoding of the current line. This stateprovides array control signals for writing of the ICSTORE array andreading of the next sequential block in the ICTAGV. If the nextsequential block is present, as soon as the current line is completed inpre-decoding, the state transfers to the Pre-decode Write state. Thearray control signals for this transfer write to the ICPDAT andICNXTBLK. If the next sequential block is not present, completingpre-decoding of the current line causes the Icache PC to increment andwriting of the new line into the ICSTORE, and restarts the pre-decodingof the new line. If there is an instruction which wraps to the new line,writing of the last line into the ICPDAT and ICNXTBLK must wait forcompletion of pre-decoding of this wrapped instruction. Duringpre-decoding, a taken branch can be detected and the state transfers tothe Pre-decode Write state.

STATE6: Pre-decode Write State

This state is a temporary state to write the ICPDAT and the ICNXTBLKbefore transfering to the Idle state or Cache Access state. If the nextblock address is present from either sequential block or taken branchaddress which is calculated by the ICPRED, then the state transfers tothe Cache Access state. The transfer provides Icache control signals forreading the array. If the taken branch address cannot be calculated bythe ICPRED, then the state transfers to the Idle state and waits for thetarget address from decoding or executing of the instruction. Forsplit-line taken-branch instruction, the pre-decode data of the previousline and the current line (ending of the taken-branch instruction) mustbe written into the ICPDAT in two clock cycles. The state machine mustremain in this state for two clock cycles.

Testability

It is important to implement testability features into Processor 500 toreduce test time, burn-in time, and increase fault coverage. TheBuild-In-Self-Test (BIST) for the arrays andAuto-Test-Pattern-Generation for the random logic are included.

BIST

The BIST is implemented with a chain of input registers to latch andwrite various patterns into the arrays and to read and compare the datainto a chain of output registers. The test pattern is serially shiftedinto the input registers, and the results are serially shifted out fromoutput registers. With 11-bit counter, the BIST registers for the Icachearrays are with these sizes:

ICSTORE--128 bits.

ICPDAT--48 bits.

ICTAGV--22 bits. (2 bits are dual-port)

ICNXTBLK--55 bits. (5 bits are dual-port)

The ICSTORE and the ICPDAT form one chain of 176 bits, and the ICTAGVand the ICNXTBLK form another chain of 77 bits. The second chain must gothrough the March C twice to access the second port. There is one outputpin for each of the chains of BIST registers. The March C algorithm has5 steps:

After the test pattern is shifted into the input registers, for everyindex from 0 to maximum count, write from input registers into thearrays, stall one cycle, then increment to next index.

For every index from 0 to maximum count, read the array, compare withthe input register, set the result register if no match, invert the testpattern in the input register, write into the array, read the array,compare with the input register, set the result register if no match,stall one cycle, and increment to next index.

For every index from 0 to maximum count, repeat the above step.

For every index from maximum count to 0, read the array, compare withthe input register, set the result register if no match, invert the testpattern in the input register, write into the array, read the array,compare with the input register, set the result register if no match,stall one cycle, and decrement to next index.

For every index from maximum count to 0, repeat the above step. Aftercompletion, the results are shifted out and reset while a new testpattern is shifted in, the 5 steps are repeated until test patterns aretested.

The BIST for the arrays are used in the following modes:

Normal BIST--Activate by holding INIT pin high and 4FLUSH pin low whilethe RESET pin is de-asserted. The March C test is run until completionand the result is latched into the EAX register. This is for testingpurpose.

Burn-in BIST--Activate by holding INIT pin high while the RESET pin isde-asserted. The March C test is continuously run concurrently with theATPG until the RESET pin is asserted. The TDO pin is driven high foreach completion of the March C test. This is to have many nodes togglefor burn-in purposes.

JTAG RUNBIST--Activated by the JTAG pins to load and execute the RUNBISTinstruction. The processor must be in reset. When the RESET pin isde-asserted, the March C test is run until completion and the result isshifted out to the TDO pin. A special mode can be activated to shift inthe test pattern and shift out the result through other pins. The RESETpin must be asserted to end the operation. This is for testing anddetecting faults.

JTAG BITMAP BIST--Activated by the JTAG pins to load and execute theBITMAP instruction. The processor must be in reset. When the RESET pinis de-asserted, the March C test is ran, and results are shifted out tothe OUTPUT pins after every compare operation. This is for detectingfaults

JTAG FLUSH REGISTER BIST--Activate by the JTAG pins to load and executethe FLUSH REGISTER instruction, the processor must be in reset. When theRESET pin is de-asserted, the input pattern from the TDI pin is shiftedinto the result registers serially out to the OUTPUT pins. The RESET pinmust be asserted to end the operation. This is for detecting faults inthe result registers.

The BIST controller generates the following five test patterns:

    ______________________________________                                                  0101 0101 0101 0101                                                           0011 0011 0011 0011                                                           1001 1001 1001 1001                                                           0000 1111 0000 1111                                                           0000 0000 1111 1111                                                 ______________________________________                                    

The Build-In Self-Test (BIST) uses the Test Application and ErrorCompression (TAEC) cells for reading and writing the arrays. Each TAECcell includes an input shift register for the test pattern and oneoutput shift register for the result. All TAEC cells are connected toform a serial shift path.

ATPG

The ATPG is implemented to test non-array blocks in the Icache. Thepurpose is to be able to reach any node in the logic. The feedback(loop) paths (state machines) must be broken with scan latches. There isa software algorithm to insert the scan latch into the logic. Thecontrol blocks in Icache should include the ATPG inputs and outputs forthe software to use.

Signal List

BSTRUN--Input from TAP indicates to start the BIST.

BSTRD--Input from TAP indicates to read the array and compare to set theresult.

BSTWR--Input from TAP indicates to write the array from input registers.

BSTRST--Input from TAP indicates to reset the counter.

BSTINCR--Input from TAP indicates to increment the counter.

BSTDIN--Input from TAP indicates the test pattern to the inputregisters. The input can be from the TDI pin or normal burn-in patterns.

FLUSHON--Input from TAP indicates flushing register mode, the resultlatch should use BSTDIN instead of the compare input for flushing theresult registers.

UPDOWN--Input from TAP indicates counting up or down.

BSTSHF1--Input from TAP indicates shifting of the master latch ofregisters.

BSTSHF2--Input from TAP indicates shifting of the slave latch ofregisters.

BSTFALSE--Input from TAP indicates to invert the test pattern.

PORTSEL--Input from TAP indicates to select the second dual port.

BSTIDOUT--Output to TAP indicates the result of the data chain from theICSTORE and ICPDAT arrays.

BSTITOUT--Output to TAP indicates the result of the data chain from theICNXTBLK and ICTAGV arrays.

BSTAMSB--Output to TAP indicates maximum count for dual port arrays.

MAXADDR--Output to TAP indicates maximum index counter.ATPGIN(15:14)--Input from dedicated pins for ATPG.ATPGOUT(15:14)--Output to dedicated pins for ATPG.

Timing

Since the clock cycle is short, reading of the cache would take thewhole clock to get data. The clock is single phase, and the arraygenerates its own self time clock. The self-time clock uses the samecache column self-time line. As the line pre-charges to a high level,the pre-charge is disabled and the array access is enabled. As the linedischarges, the row driver and senamp are disabled. In one embodiment,the pre-charge takes 1.7 ns and the current timing for TAGHIT from theself-time clock with 64 rows is 2.8 ns or a total time of 4.5 ns fromrising edge of ICLK. The reading data is 2.0 ns from the self-time clockwith 64 rows or 0.8 ns before the rising edge of ICLK. The ICSTORE canbe built with larger arrays, 128 rows by 256 columns, readinginstructions would take all of 4.5 ns ICLK in this case. Other arrays,ICTAGV, ICPRED, and ICNXTBLK, are 64 rows. The align logic in the ICPDATtakes 6-7 gates, and the shifting of X86 instruction bytes to the decodeunit can be done by the middle of the second ICLK. The Processor 500instructions should allow the decode units at least 2.5 ns in the secondICLK for calculation of the linear address.

cycle 1: ICFPC, muxing new PC, pre-charge, and access all arrays

cycle 2: Compare tags, aligning logic's from pre-decode, setup branchprediction, and muxing instructions to decode units on IB buses.

cycle 2.5: Displacement linear address calculation. Fast decoding forregister operands and validating of the linear address, and fastdecoding for non-conditional branch.

If the predicted branch from the ICNXTBLK is taken, the new PC will taketwo clock cycles to update in the ICFPC. The speculative way-predictiontakes 2 gates for set decoding, 3 gates for muxing of success index toICFPC, and 2 gates in row decoding.

The timing for instructions from external memory is as follows:

cycle 1: Latch data from INSB bus to pre-fetch buffer and mux onto IBbuses to ICPRED in next clock; the data on IB buses are held untilpre-decode is completed. Write data into cache.

cycle 2: Decode opcode and prefix from the byte pointer. Decoding takes2 clock cycles. If there is prefix, then restart the decoding of opcodein the next cycle.

cycle 3: Decode opcode. Send pre-decode data to ICPDAT and allow thealign logic to select the instruction on IB buses to decode units

cycle 4: Send instruction from IB buses to decode units on IBDx buses.The IBDx buses should have the same timing as reading from the array.

The MROM interface requires a different timing:

ICLK3: Detect MROM instruction and send the byte position to MROMinterface.

ICLK4: Decode prefixes and generate MROM entry point.

ICLK5: Decode instruction.

ICLK6: Decode instruction and latch all field of instructions intoglobal registers.

ICLK6: MROM reads global registers and sends micro-instruction to decodeunits by mid cycle.

Layout

With a short clock cycle, the size of the arrays are limited to 128 rowsby 256 columns for single-port RAM arrays which read or write indifferent clock cycles and not in speed path. For dual-port RAM arraysor faster read timing, the 64 rows by 256 columns array is preferred.The array sizes are based on the single port RAM cell of 10.25 u×6.75 u,and the dual port RAM cell of 10.25 u×14.5 u. The arrays in the ICACHEare layout as shown in FIG. 27:

ICSTORE--2048 lines of 128 bits, 8 sets of 128×256, 1312 u×1728 u,single.

ICPREDAT--2048 lines of 48 bits, 8 sets of 64×192, 656 u×1296 u, single.

ICTAGV--2048 lines of 24 bits, 3 sets of 64×224, 656 u×1512 u, single,and 1 set of 64×96, 656 u×1392 u, dual.

ICNXTBLK--2048 lines of 51 bits, 5 sets of 64×256, 656 u×1728 u, single,1 set of 64×192, 656 u×1296 u, single, 1 set of 64×96, 656 u×1392 u,dual, and 1 set of 64×64, 656 u×928 u, dual.

ICTAGV includes a of 20-bit tag, a 1-bit valid, a 3-bit status, and a3-bit way-prediction. The tag and valid are single-port RAM, and thestatus and way-prediction are dual-port RAM. The ICNXTBLK does notinclude the global branch prediction. A suggested layout for Icache isshown in FIG. 27.

Overview of the Processor 500 Idecode

This section describes the instruction decode organization. Theinstruction decoding is directly for X86 instructions; there will be noROPs except for a few instructions. The X86 variable-length instructionsfrom the Icache are sent to the fixed-length decode units. Up to 4instructions can be decoded and dispatched in one clock cycle. Thedecode units are identical. The operand and flag are decoded in theIcache for the critical speed path. The instruction decoding takes 2clock cycles, in the first cycle the operand and flag dependencychecking is done and multiplexed to the functional units in the secondcycle. The operand addresses access the register file and the FIROB. Theoperand address includes the X86 registers and temporary registersgenerated by the MROM. The FIROB will have to check for dependencies.The instruction decoding generates direct controls to the operation ofthe functional unit, thus eliminating decoding in the functional units.The global controls of the decode units include a mechanism to stall theline due to serialization and limitations of the load/store buffers, theFIROB, and the reservation stations. The MROM interfacing is sentdirectly from the Icache and the MROM instructions go through the muxesin the Icache to decode units. A few exceptions for MROM instructionsare special registers, floating point instructions, temporary registers,and serialization.

Top Level of Idecode

The Idecode has 4 identical decode units. The instructions from Icacheand MROM have the same format. Operands and flags are decoded in theIcache or MROM before the instructions are dispatched to the decodeunits. The MROM may send the micro-instructions with simpler opcodes tothe decode units. Decode units decode the micro-instructions separatelyand multiplex with fast-path instructions to functional units. Prefixesare pre-decoded and sent on separate buses. The format of instructionsdispatched to the decode unit includes one encoded prefix byte, whichmay not be valid, speculative register operands and flags, 2-dispatchposition control bits, 11-bit opcode, 3-bit constant, up to 4 bytes ofdisplacement, and 4 bytes of immediate field. Since the Icache validatesand partially decodes the instructions before sending to the decodeunits, a valid bit is issued by the Icache for valid instructions. Ifthe valid is not set, the instruction is dispatched as NOOP. For2-dispatch position instructions, the indication from Icache causes thedecoding of the same instruction to two issue positions. Idecode has 2clock cycles, the first cycle is for register file accessing and FIROBdependency checking, and the second cycle is for operand steering of theoperands and dependency tags to the functional units. The opcodedecoding takes 2 clock cycles to generate detail control signals for thefunctional units. Since the register operands and the immediate fieldare speculative, the opcode decoding is needed for validation andindication of the direction for the operands and the size of theimmediate field if any. The immediate field should be sign-extended asit is sent to the functional unit.

To handle the PC properly, the Icache provides 2 PC's for each set ofdispatched instructions per clock cycle. The first case is when theinstructions cross the page boundary, and the second case is when thebranch target instruction is dispatched along with the branchinstruction. The limitations for branch in each 4 instructions are onetaken branch instruction and two branches per line.

The Idecode can be divided into 4 blocks:

Opcode decoding for functional unit which can be done in 2 clock cycles.

Opcode decoding for registers and immediate field, generating controlsfor the operands and flags, and selecting the target address for branchinstruction to reservation stations which should be done in 1 clockcycles.

Decoding of other instruction fields such as lock, segment registercontrols, special register, and floating point unit.

Global control of the pipeline, serialization, and stalling conditions.

Signal List

IRESET--Global signal used to reset all decode units. Clear all states.

EXCEPTION--Global signal from the FIROB. Used to indicate that aninterrupt or trap is being taken. Effect on Idecode is to clear allinstructions in progress.

BRNMISP--Input from the Branch execution of the FU indicates a branchmis-prediction. The Idecode clears all instructions in progress.

ROBEMPTY--Input from the FIROB indicates the FIROB is empty.

ROBFULL--Input from the FIROB indicates the FIROB is full.

CS32X16--Input from the LSSEC indicates the size of the code segmentregister.

SS32X16--Input from the LSSEC indicates the size of the stack segmentregister.

MVTOSRIAD--Input from SRB, indicates a move to IAD special register,Idecode needs to check its pointer against the pointer driven on IAD.

MVFRSRIAD--Input from SRB, indicates a move from IAD special register,Idecode needs to check its pointer against the pointer driven on IAD.

MVTOARIAD--Input from SRB, indicates a move to IAD special registerarray, Idecode needs to check its pointer against the pointer driven onIAD.

MVFRARIAD--Input from SRB, indicates a move from IAD special registerarray, Idecode needs to check its pointer against the pointer driven onIAD.

RSFULL--Input from the functional units indicates the reservationstation is full.

HLDISP(1:0)--Input from Icache indicates all instructions of the first(bit 0) and/or the second (bit 1) 8-byte of the current line has beendispatched to decode units.

ICPC1(31:0)--Input from Icache indicates the current line PC of thefirst instruction in the 4 issued instructions to pass along with theinstruction to the FIROB.

ICPC2(31:0)--Input from Icache indicates the current line PC of a secondinstruction which cross the 16-byte boundary or branch target in the 4issued instructions to pass along with the instruction to the FIROB.

ICPOSx(3:0)--Input from Icache to decode units indicates the PC's byteposition of the instruction.

ICBTAG1(3:0)--Input from Icache indicates the position of the firsttarget branch instruction with respect to the global shift register incase of branch mis-prediction. The branch can be taken or non-taken,branch tag must be sent with all branch instruction.

ICBTAG2(3:0)--Input from Icache indicates the position of the secondtarget branch instruction with respect to the global shift register incase of branch mis-prediction. The branch can be taken or non-taken,branch tag must be sent with all branch instruction.

UNJMP(3:0)--Input from Icache indicates the unconditional branchinstruction needs to calculate target address.

BRNTKN(3:0)--Input from Icache indicates which decode unit has apredicted taken branch. The operand steering uses this signal to latchand send BTADDR(31:0) to the functional unit.

BRNINST(3:0)--Input from Icache indicates which decode unit has a globalbranch prediction. The operand steering uses this signal to latch andsend ICBTAG1(3:0) and ICBTAG2(3:0) to the functional units.

ICPREF(7:0)--Input from Icache and MROM indicates the encoded prefixbyte. The two most significant bits are repeat prefixes for MROM.

IC2ROPn(9:0)--Input from Icache to decode unit n indicates 2-dispatchpositions instruction. Bit 3 indicates the first rop or second rop ofthe 2-dispatch positions instruction, bit 2 indicates POP instruction,bit 1 indicates the MUL instruction, and bit 0 indicates the SIB-byteinstruction.

IB2(191:0)--Input from Icache indicates the combined instruction linefor reading the displacement and immediate field using pointers.

ICVALI(3:0)--Input from Icache indicates valid instructions. NOOP isgenerated for invalid instruction.

ICnOPC(7:0)--Input from Icache to decode unit n indicates the opcodebyte.

ICnEOP(2:0)--Input from Icache to decode unit n indicates the extendedopcode field.

ICnSS(1:0)--Input from Icache to decode unit n indicates the scalefactor of the SIB byte.

DISPTRn(6:0)--Input from Icache to decode unit n indicates thedisplacement pointer and size. Bits 2:0 is the pointer to the 8-byteblock, bit 6:5 is the size, and bit 4:3 indicates which 8-byte block.Bit 6:5=00 indicates no displacement.

IMMPTRn(4:0)--Input from Icache to decode unit n indicates thedisplacement pointer and size. Bits 2:0 is the pointer to the 8-byteblock, and bit 4:3 indicates which 8-byte block. Decoding of all opcodesis needed to detect immediate field.

MROMEN--Input from MROM indicates the micro-instructions is sent toIdecode instead of the Icache.

CONSTn(2:0)--Input from Icache to decode unit n indicates the constantfor add/substract to ESP of the two-dispatch position instruction.

ICMROM--Output to MROM indicates the current instruction is MROM. TheMROM instruction may take two cycles to read the IB, ICEND, and ICFUNC.

ICPC1TAR--Input from Icache indicates is ICPC1 a branch target of aprevious instruction which is a predicted taken branch instruction.

ICPC2TAR--Input from Icache indicates is ICPC2 a branch target of aprevious instruction which is a predicted taken branch instruction.

ICPC1(31:0)--Input from Icache indicates the current line PC of thefirst instruction in the 4 issued instructions to pass along with theinstruction to FIROB.

ICPC2(31:0)--Input from Icache indicates the current line PC of a secondinstruction which cross the 16-byte boundary or branch target in the 4issued instructions to pass along with the instruction to FIROB.

ICPOSn(4:0)--Input from Icache to decode unit n indicates the PC's byteposition of the next instruction. Bit 4 indicates the next instructionis on the next line.

BTAGlN(3:0)--Output indicates the position of the first target branchinstruction for a new line with respect to the global shift register incase of branch mis-prediction.

BTAG2N(3:0)--Output indicates the position of the second target branchinstruction for a new line with respect to the global shift register incase of branch mis-prediction.

BTAKEN1(1:0)--Input from Icache to decode units and ICFPC indicates apredicted taken branch instruction from PTAKEN, BVAL1. Bit 0 is the lastline and bit 1 is new line.

BTAKEN2(1:0)--Input from Icache to decode units and ICFPC indicates apredicted taken branch instruction from PTAKEN, BVAL2. Bit 0 is the lastline and bit 1 is new line.

ICERROR--Input from Icache indicates an exception has occurred on aninstruction pre-fetched, the type of exception (TLB-miss, page-fault,illegal opcode, external bus error) will also be asserted.

BTADDR(31:0)--Output to functional units indicates the taken branchtargets from either the branch prediction (IBTARGET from Icache) orunconditional branch. The functional units need to compare to the actualbranch target.

BRNTKN(3:0)--Output indicates which decode unit has a predicted takenbranch. The operand steering uses this signal to latch and sendBTADDR(31:0) to the functional unit.

BRNINST(3:0)--Output indicates which decode unit has a global branchprediction. The operand steering uses this signal to latch and sendBTAG1(3:0) and BTAG2(3:0) to the functional units.

IDxIMM(2:0)--Output indicates the immediate size information. 01-byte,10-half word, 11-word, 00-not use. Bit 2 indicates (0) zero or (1) signextend.

IDxDAT(1:0)--Output indicates the data size information. 01-byte,10-half word, 11-word, 00-not use.

IDxADDR--Output indicates the address size information. 1-32 bit, 0-16bit.

IDxLOCK--Output indicates the lock prefix is set for this instructionfor serialization.

DxUSE1(1:0)--Output to FIROB and register file indicates the type ofoperand being sent on operand 1 for decode units. Bit 1 indicates sourceoperand, and bit 0 indicates destination operand.

DxUSE2--Output to FIROB and register file indicates source operand.

INSDISP(3:0)--Indicates that the instruction in decode unit is valid, ifinvalid, NOOP is passed to FIROB.

IMDIWx(31:0)--Output indicates the 32-bit displacement or immediatefield of the instruction to pass to the functional units.

IMDINx(7:0)--Output indicates the 8-bit displacement or immediate fieldof the instruction to pass to the functional units.

USEIDW(3:0)--Output indicates the type used in IMDIWx buses.

USEIDN(3:0)--Output indicates the type used in IMDINx buses.

INSLSxB(5:0)--Output from decode units indicates the prefix values. bit5--data size, bit 4--address size, bit 3--lock, bit 2:0--segmentregisters.

REFRESH2--Output indicates current line of instructions will berefreshed and not accept new instructions from Icache.

INSOPxB(11:0)--Output indicates the type of instructions beingdispatched, this is the decoded information for the functional units toexecute.

Instruction Opcode Decoding

The instruction decoding has 1.5 clock cycles for decoding, therebyallowing time to route the output to functional units. The output is awide bus with direct commands for the functional units to execute theinstruction. The current listing for functional unit includes 3-bitprotocol, 6-bit opcode, 7-bit p/g for alu, 3-bit size, 4-bit segmentregister select, and 3-bit flag. Except for the p/g for alu, all othercontrol bits for the functional unit must be further decoded intosingle-bit control. The size and segment register select are decoded ina later section. The listing of the actual binary is done by goingthrough the X86 instruction one-by-one.

First 6 bits of decoding:

    ______________________________________                                        000001    ADD         add                                                     000011    OR          or                                                      000101    AND         and                                                     000111    SUB         subtract                                                001001    XOR         exclusive or                                            001011    ANDN        nand                                                    001101    XNOR        exclusive nor                                           001111    CONST       constant                                                000000    ADDC        add with carry                                          000010    SUBB        subtract                                                000100    DFADD       directional add                                         000110    INT         interrupt                                               001000    INTO        interrupt on overflow                                   001010    DIV0        initial divide step                                     001100    DIV         divide step                                             001110    DIVL        last divide step                                        010000    DIVREM      remainder                                               010010    DIVCMP      divide compare                                          010100    DIVQ        quotient                                                010110    IDIVSGN     signed divide signs                                     011000    IDIVCMP     signed divide compare                                   011010    IDIVDEND0   signed divide dividend LSW                              011100    IDIVDEND1   signed divide dividend MSW                              011110    IDIVSOR     signed divide divisor                                   011111    IDIVQ       signed divide quotient                                  100000    RQL         rotate left                                             100001    ROR         rotate right                                            100010    SHL         shift logical left                                      100011    SHR         shift logical right                                     100100    SAR         shift arithmetic right                                  100101    SHLD        shift left double                                       100110    SHRD        shift right double                                      100111    SETFC       set funnel count                                        101000    EXTS8       sign extend 8 bit operand                               101001    EXTS16      sign extend 16 bit operand                              101100    MTFLAGS     store AH into flags                                     101101    CONSTHZ     move lower constant into                                                      upper, zero lower                                       101110    BTEST       bit test                                                101111    BTESTS      bit test and set                                        110000    BTESTR      bit test and reset                                      110001    BTESTC      bit test and compliment                                 110010    BSF         bit scan forward                                        110011    BSR         bit scan reverse                                        110100    BSWAP       byte swap                                               110101    SHRDM       shift right double                                                            microcode                                               110110    RC0         initialize rotate carry                                 110111    RCL         rotate carry left by 1                                  111000    RCR         rotate carry right by 1                                 111001    MTSRRES     move to special register                                                      over result bus                                         111010    MFSRRES     move from special register                                                    over result bus                                         111011    MTSRSRB     move to special register                                                      over SRB bus                                            111100    MFSRSRB     move from special register                                                    over SRB bus                                            111101    MTARSRB     move to cache array over                                                      SRB bus                                                 111110    MFARSRB     move from cache array over                                                    SRB bus                                                 ______________________________________                                    

Second 6 bits of decoding:

    ______________________________________                                        000000    JMPB        jump if below CF=1                                      000001    JMPNB       jump if not below CF=0                                  000010    JMPA        jump if above CF=0 & ZF=0                               000011    JMPNA       jump if not above CF=1 or                                                     ZF=1                                                    000100    JMPO        jump if overflow OF=1                                   000101    JMPNO       jump if not overflow OF=0                               000110    JMPZ        jump if zero ZF=1                                       000111    JMPNZ       jump if not zero ZF=0                                   001000    JMPS        jump if sign SF=1                                       001001    JMPNS       jump if not sign SF=0                                   001010    JMPP        jump if parity PF=1                                     001011    JMPNP       jump if not parity PF=0                                 001100    JMPL        jump if less SF<>OF                                     001101    JMPGE       jump if greater or equal                                                      SF=OF                                                   001110    JMPLE       jump if less or equal                                                         SF<>OF or ZF=1                                          001111    JMPG        jurnp if greater SF=OF and                                                    ZF=0                                                    010000    SETB        set if below CF=1                                       010001    SETNB       set if not below CF=0                                   010010    SETA        set if above CF=0 & ZF=0                                010011    SETNA       set if not above CF=1 or                                                      ZF=1                                                    010100    SETO        set if overflow OF=1                                    010101    SETNO       set if not overflow OF=0                                010110    SETZ        set if zero ZF=1                                        010111    SETNZ       set if not zero ZF=0                                    010000    SETS        set if sign SF=1                                        011001    SETNS       set if not sign SF=0                                    011010    SETP        set if parity PF=1                                      011011    SETNP       set if not parity PF=0                                  011100    SETL        set if less SF<>OF                                      011101    SETGE       set if greater or equal                                                       SF=OF                                                   011110    SETLE       set if less or equal SF<>OF                                                   or ZF=1                                                 011111    SETG        set if greater SF=OF and                                                      ZF=0                                                    100000    SELB        move if below CF=1                                      100001    SELNB       move if not below CF=0                                  100010    SELA        move if above CF=0 & ZF=0                               100011    SELNA       move if not above CF=1                                                        ZF=1                                                    100100    SELO        move if overflow OF=1                                   100101    SELNO       move if not overflow OF=0                               100110    SELZ        move if zero ZF=1                                       100111    SELNZ       move if not zero ZF=0                                   101000    SELS        move if sign SF=1                                       101001    SELNS       move if not sign SF=0                                   101010    SELP        move if parity PF=1                                     101011    SELNP       move if not parity PF=0                                 101100    SELL        move if less SF<>OF                                     101101    SELGE       move if greater or equal                                                      SF=OF                                                   101110    SELLE       move if less or equal                                                         SF<>OF or ZF=1                                          101111    SELG        move if greater SF=OF and                                                     ZF=0                                                    110000                                                                        110001    CONSTPC     move from EIP over DPC                                  110010    JMP         relative jump                                           110011    JMPI        absolute jump                                           110100    JMPNU       absolute jump, no                                                             prediction update                                       110101    JMPIFAR     absolute far jump                                       110110    JMPRZ       jump if A.sub.-- OP == 0                                110111    JMPNRZ      jump if A.sub.-- OP |= 0                                111000    JMPNRZZ     jump if A.sub.-- OP |= 0 & ZF==1                        111001    JMPNRZNZ    jump if A.sub.-- OP |= 0 & ZF==0                        111010    JNPRS       jump if A.sub.-- OP msb==1                              111011    JMPRNS      jump if A.sub.-- OP msb==0                              111100                                                                        111101                                                                        111110                                                                        111111                                                                        ______________________________________                                    

One exception in the above decoding is the reversed subtract which wouldbe sent as subtract instruction. The MROM instruction will try to usethe X86 opcode format. Some operations are not possible in the fast-pathinstructions such as floating point, string, integer divide, specialregister, and processor control. These MROM instructions will havedifferent opcodes which need to be recognized by Idecode.

Signal List

ICPREF(7:0)--Input from Icache and MROM indicates the encoded prefixbyte. The two most significant bits are repeat prefixes for MROM.

IC2ROPn(9:0)--Input from Icache to decode unit n indicates 2-dispatchpositions instruction. Bit 3 indicates the first rop or second rop ofthe 2-dispatch positions instruction, bit 2 indicates POP instruction,bit 1 indicates the MUL instruction, and bit 0 indicates the SIB-byteinstruction.

ICVALI(3:0)--Input from Icache indicates valid instructions. NOOP isgenerated for invalid instruction.

ICnOPC(7:0)--Input from Icache to decode unit n indicates the opcodebyte.

ICnEOP(2:0)--Input from Icache to decode unit n indicates the extendedopcode field.

PROTOCOLn(3:0)

OPCTLn(5:0)

PGALUn(6:0)--Output indicates the type of instructions being dispatched,this is the decoded information for the functional units to execute.

Decoding for Registers and Flags

The opcode should also be decoded for the immediate field, thevalidation and direction of the operands to the FIROB and the registerfile. The decoding of the register direction and validation should beless than 1 clock cycle. During fetching, the register operands arespeculatively decoded and sent to the FIROB for dependency checking. TheFIROB's dependency checking may be in a critical speed path, and thedecoding of the register direction and validation may be implemented inthe early decoding in the Icache. Two register operands are sent to theFIROB. The Idecode sends the operand data to the functional units in thecorrect order of A and B. The A operand is the first operand which canbe both destination and source. The B operand is the second operandwhich is source data. The immediate field co-exists with the B operandon three instructions, IMUL, SHLD, and SHRD. SHLD and SHRD are MROMinstructions which will be sent as shifting one bit at a time. Theexception is with the IMUL instruction. The first operand is only fordestination which is not needed by the functional unit. The secondoperand will be sent on the A operand and immediate is on B operand. Thedata and address sizes and the sign-extended information for alloperands and immediate data must also be decoded in this block. Datafrom Idecode is arranged on three 32-bit buses and one constant bus tothe functional units:

    ______________________________________                                        From Idecode        To Functional Unit                                        ______________________________________                                        First operand       32-bit A operand bus                                      Second operand (not IMUL)                                                                         32-bit B operand bus                                      Second operand - IMUL                                                                             32-bit A operand bus                                      Immediate           32-bit B operand bus                                      Predicted Branch Target                                                                           32-bit A operand bus                                      EIP/second operand  32-bit B operand bus                                      Displacement        32-bit Displacement bus                                   Constant            4-bit Constant bus                                        ______________________________________                                    

For branch instructions, the target PC is sent on A operand bus becauseit is possible to have the immediate field (RETURN instructions) anddisplacement field. All predicted taken branch instructions haveindication and branch tag to keep in the FIROB. Few non-taken branchinstructions are not detected in branch prediction, the FIROB keepstrack of the sequential PC.

This block also decodes the load/store, special register, and floatingpoint operations, and serialization which are needed global control ofthe decode units.

Signal List

ICBTAG1(3:0)--Input from Icache indicates the position of the firsttarget branch instruction with respect to the global shift register incase of branch mis-prediction. The branch can be taken or non-taken,branch tag must be sent with all branch instruction.

ICBTAG2(3:0)--Input from Icache indicates the position of the secondtarget branch instruction with respect to the global shift register incase of branch mis-prediction. The branch can be taken or non-taken,branch tag must be sent with all branch instruction.

UNJMP(3:0)--Input from Icache indicates the unconditional branchinstruction needs to calculate target address.

BRNTKN(3:0)--Input from Icache indicates which decode unit has apredicted taken branch. The operand steering uses this signal to latchand send BTADDR(31:0) to the functional unit.

BRNINST(3:0)--Input from Icache indicates which decode unit has a globalbranch prediction. The operand steering uses this signal to latch andsend ICBTAG1(3:0) and ICBTAG2(3:0) to the functional units.

IB2(191:0)--Input from Icache indicates the combined instruction linefor reading the displacement and immediate field using pointers.

ICVALI(3:0)--Input from Icache indicates valid instructions. NOOP isgenerated for invalid instruction.

ICnOPC(7:0)--Input from Icache to decode unit n indicates the opcodebyte.

ICnEOP(2:0)--Input from Icache to decode unit n indicates the extendedopcode field.

ICnSS(1:0)--Input from Icache to decode unit n indicates the scalefactor of the SIB byte.

DISPTRn(6:0)--Input from Icache to decode unit n indicates thedisplacement pointer and size. Bits 2:0 is the pointer to the 8-byteblock, bit 6:5 is the size, and bit 4:3 indicates which 8-byte block.Bit 6:5=00 indicates no displacement.

IMMPTRn(4:0)--Input from Icache to decode unit n indicates thedisplacement pointer and size. Bits 2:0 is the pointer to the 8-byteblock, and bit 4:3 indicates which 8-byte block. Decoding of all opcodesis needed to detect immediate field.

CONSTn(2:0)--Input from Icache to decode unit n indicates the constantfor add/substract to ESP of the two-dispatch position instruction.

ICPCLTAR--Input from Icache indicates is ICPC1 a branch target of aprevious instruction which is a predicted taken branch instruction.

ICPC2TAR--Input from Icache indicates is ICPC2 a branch target of aprevious instruction which is a predicted taken branch instruction.

ICPC1(31:0)--Input from Icache indicates the current line PC of thefirst instruction in the 4 issued instructions to pass along with theinstruction to FIROB.

ICPC2(31:0)--Input from Icache indicates the current line PC of a secondinstruction which cross the 16-byte boundary or branch target in the 4issued instructions to pass along with the instruction to FIROB.

ICPOSn(4:0)--Input from Icache to decode unit n indicates the PC's byteposition of the next instruction. Bit 4 indicates the next instructionis on the next line.

IDxIMM(2:0)--Output to indicates the immediate size information.01-byte, 10-half word, 11-word, 00-not used. Bit 2 indicates (0) zero or(1) sign extend.

IDxDAT(1:0)--Output to indicates the data size information. 01-byte,10-half word, 11-word, 00-not used.

IDxADDR--Output to indicates the address size information. 1-32 bit,0-16 bit.

DxUSE1(1:0)--Output to FIROB and register file indicates the type ofoperand being sent on operand 1 for decode units. Bit 1 indicates sourceoperand, and bit 0 indicates destination operand.

DxUSE2--Output to FIROB and register file indicates source operand.

INSDISP(3:0)--Indicates that the instruction in decode unit is valid, ifinvalid, NOOP is passed to FIROB.

Segment Register and Special Control

In general, writing to special registers and cache array causesserialization after dispatching of the instruction. Reading of specialregister and cache array does not cause serialization. Cache arrayaccess is from MROM instructions for testing and debugging purposes.Most special register access is also from MROM instructions; few MOVEinstructions specify the special register directly. Serialization is amajor degradation in performance. Implementation of a special registerbuffer in the load/store section reduces the serialization as will bediscussed in a later section. For special register access, thedestination register of the instruction is the indication of specialregister (not a real register in the register file), the specialregister address and control information are encoded in the immediatefield of the MROM instruction. The functional unit will pass theimmediate field directly to the special register buffer. The immediatefield is decoded in the special register to generate read/write to theappropriate special register or cache array. Fast-path instruction whichread/write to the special register must provide the same format in theimmediate field as the MROM to the functional unit.

The prefix of the instruction is decoded during fetching. The segmentregister override is encoded as a prefix byte. The MROM instructionprovides the ES reference for the string operation. All accesses tosegment registers must be detected in decoding to generate controls tothe load/store section. The percentage of writing to some segmentregister is significant to the performance of Processor 500. To furtherreduce the penalty of serialization due to the writing of segmentregisters, control signals and dependency status bits are generated. Inthe next section, a detailed implementation of this technique isdiscussed.

The floating point unit is implemented as a co-processor. The MROMprovides the same format for the floating point instructions in 4 issuepositions. The first three issue positions are for the possible 80-bitload/store access of the floating point instructions. For 32-bit or64-bit floating point operations, the issue position is filled withNOOP. The last issue position is the actual opcode of the floating pointinstruction. The floating point opcode is in the immediate field of theMROM instruction. Idecode treats the floating point instruction as aspecial register instruction; the immediate field is passed from specialregister block to the floating point unit. The floating point unitlatches and loads data from the LSSEC and send the store data throughthe functional unit to the LSSEC.

Special-Register Instructions

There are two types of special-register (SR) instructions. One is fortesting/debugging purpose, and the other is for normal execution. Thetesting/debugging SR instructions are for the cache arrays, it is notimportant to speed up these instructions. Implementation of theserialized instruction is as followed:

SR instructions can be issued from any dispatch position.

The SR address is sent to the functional unit in the Immediate field.

Read from SR instructions are not serialized.

Up to 4 SR instructions can be in execute. The load/store specialregister (LSR) buffer has 4 entries. The LSR buffer is global to boththe SRB and LSR. Idecode will communicates with the LSR to allocate anentry when a SR instruction is dispatched. This is the sameimplementation as with the load/store buffer.

Write to SRs except to DS, GS, ES, and FS are serialized. Theserialization is to dispatch the write to SR instruction, stall allother instructions, and wait for LSR to notify the completion of theinstruction. The write to LSR instruction must execute in program order(same as STORE instructions)

Write to DS, GS, ES, and FS instruction sets a dependency bit for eachof the four segment register when dispatching. Four global WRxS signalsto all functional units indicate the write to the segment register isoutstanding. The LSR will reset the dependency bit and clear WRxS fromexecution of the write to SR instruction. Any read to DS/GS/ES/FSinstruction after the write will have a status read dependency bit (RD₋₋DEP₋₋ xS). Instructions with RD₋₋ DEP₋₋ xS set, must wait in reservationstation until WRxS is cleared. The read to GS/ES/FS instruction includes(1) Prefix segment override, (2) Move from segment register, and (3)String operation (ES only). The string operation will have indicationfrom MROM. A second write to segment register with WRxS set, must stallin decode.

Signal List

IMMPTRn(4:0)--Input from Icache to decode unit n indicates thedisplacement pointer and size. Bits 2:0 is the pointer to the 8-byteblock, and bit 4:3 indicates which 8-byte block. Decoding of all opcodesis needed to detect immediate field.

MROMEN--Input from MROM indicates the micro-instructions is sent toIdecode instead of the Icache.

IDxIMM(2:0)--Output to indicates the immediate size information.01-byte, 10-half word, 11-word, 00-not use. Bit 2 indicates (0) zero or(1) sign extend.

INSLSxB(5:0)--Output from decode units indicates the prefix values. bit5--data size, bit 4--address size, bit 3--lock, bit 2:0--segmentregisters.

Global Control of Decode Units

The decode units, can decode instructions, generate operand addresses,and dispatch to the functional units independent from each other. Thereare a few exceptions where global control is needed. With the extrapipeline stage before the alignment, some decoding of the instructionsis done before sending to the decode units. Early decoding includes MROMinstruction, SIB-byte instruction, 2-dispatch position instructions,multi-prefix instruction, and branch taken instructions. Duringdecoding, partial dispatching of a line can happen for serialization,dependency, and move-to-special-register. Conditions to halt the line ofinstructions before dispatching to the functional units in the next ICLKare the reservation stations full, the FIROB full, and the Load/Storebuffer full. These halt conditions will stop the pipeline in the decoderfrom advancing. For serialization, the instructions can occupy theentries in the FIROB and wait for ROBEMPTY to dispatch the instructionsto functional units. Most of the serialization conditions is provided bythe MENG, few serialized instructions must be decoded from fast-pathinstructions. The two clock cycle in decode units are pipelined, theappropriate pipeline control such as HOLD and REFRESH are needed for thedecode units and Icache.

Stalling and Partially Dispatching of Instructions

Each stage of the pipeline has a latch and the instructions can berefreshed. In the first stage, the stalling conditions for the operandpointers and instructions from Icache to decode units are:

If the FIROB is full, the decoding is stalled until the FIROB can acceptanother line of instruction.

In the next stage, the line in the FIROB must be allocated, the stallingconditions for the operand data to remain on the buses are:

If there is a narrow-to-wide dependency; i.e. the read operand is 32-bitand the previous destination operand is 8-bit, the decoding is stalleduntil the FIROB retires the previous destination entry. Instructions maybe partially dispatched, the FIROB invalidates instructions from thenarrow-to-wide dependency. The Idecode invalidates instructions beforethe narrow-to-wide dependency and re-issues the instructions to FIROB. Asimilar procedure is needed for the second group of flag dependencychecking.

Dependency is detected for the third group of flags. If the load/storebuffer is full, the decoding is stalled until the load/store buffer isavailable. Partial line dispatching can be done instead of stalling thewhole line. This will be discussed in a later section.

If the special register buffer is full, the decoding is stalled untilthe load/store buffer is available. This was discussed in an earliersection.

Decoding is stalled on write to special registers and cache array exceptfor write to DS, ES, FS, and GS. This was discussed in an earliersection.

If any set of reservation stations is full, the decoding is stalleduntil the reservation station is available.

Serialized instructions.

Instruction Serialization

Serialization is controlled by MROM and the decode units. On someprocessors, the ROB must be empty before the instructions can bedispatched from the decode units, and the ROB must be empty again beforethe next instruction can be dispatched. The cost is about 6 cycles toexecute a single instruction. The performance degrades tremendously byserialization. On Processor 500, serialization will be minimized, mostfrequent serialization instructions will be in the fast path. Theserialization can be categorized into serialized instructions, exceptioninstructions, break-point instructions, and special registerinstructions.

Serialized instructions which must be handled by the decode units:

INVD--Dispatch the instruction to invalidate the data/instruction cacheand wait for the FIROB re-fetch the next instruction.

HALT--Dispatch the instruction to the FIROB and wait for interrupt.

WAIT--Dispatch the instruction to the FIROB and wait.

Instruction Breakpoints and Tracing

When enabled, instruction breakpoint check instructions are insertedbefore each instruction by the decode unit. A hardwired input todispatch position zero is serially dispatched before every instruction.The breakpoint instructions go to the LSSEC to check for breakpoint.

Handling of Load/Store Instructions

The load/store section implements a finite size load/store buffer. Therewould be cases that the buffer is full and creates a stall in functionalunits. To avoid stalling in the functional units, the decode will notdispatch the current line of instructions if there is not enough spacein the load/store buffer to handle the load/store instructions of thecurrent line. The decode units have more time to make this decision thanthe functional units.

At dispatch, the decode units send the load/store information to theload/store section. The information includes the current FIROB line,data dependency tags, and load/store type LSTYPE(1:0):

00: No load/store

01: Load operation

10: Store operation

11: Both Load and Store operations

If the instruction has a memory reference, then the load/store typeshould be set. The linear address and/or data will be sent to theload/store buffer from the functional units or reservation station at alater time. The load/store buffer allocates entries for the dispatchedinstructions. The store operation should get a slot in the load/storebuffer and the load instruction increases a counter to keep track of thenumber of load in the executing stage. A 3-bit count is sent to thedecode units to indicate the number of empty entries in the load/storebuffer. The decode units will dispatch a line of instructions only ifthe number of load/store instructions in the line is less than or equalto the empty entries in the load/store buffer.

Signal List

LSCNT(2:0)--Input from LSSEC indicates the number of empty entries inthe load/store buffer.

RSFULL--Input from functional units indicates that the reservationstations are full. This signal is the OR of the 6 functional units

ROBFULL--Input from FIROB indicates the FIROB is full.

REFRESH2--Output indicates that the operand pointer to the register fileand the FIROB will be refreshed and not accept new operand.

REFRESH1--Output indicates current line of instructions will berefreshed and not accept new instructions from Icache.

IDPREF(5:0)--Output from 2-cycle prefix decode to decode units indicatesthe prefix values. bit 5--data size, bit 4--address size, bit 3--lock,bit 2:0--segment registers.

IDSIB(3:0)--Output to FIROB indicates which decode unit has the SIB-byteinstruction. The FIROB should generate dependency on the previousinstruction of the last line in the same issue position.

Timing

The Idecode has 2 pipeline stages:

first stage: Decode instruction opcode for functional units. Decoderegister direction and validation for operands to dispatch to functionalunits. Dependency checking in the FIROB.

second stage: Operand steering for tags and operand data to thefunctional units. Complete decoding for instruction and send tofunctional units.

Layout

The Idecode includes six decode unit blocks. The decode unit 0 and 6 areslightly different because of passing part of the instruction fromdifferent lines. The global blocks are: MROM interface unit, the prefixdecoding and control for 2-cycle Processor 500 fast-path instructions,the return stack and controls for branch instructions, and globaldecoding controls. The MROM interface unit includes global registersaccessible by MROM instruction. FIG. 28 is a block diagram of theIDECODE layout.

Dispatch and Issue Logic

For this section, dispatch means sending a new set of up to fourpseudo-opcodes with their tags and operands to the operand steering unitand then to the reservation station. Issue means sending the sourceoperands to the FNCU for execution. The term pseudo-opcode is used sincefast path x86 instructions are sent as either one or two side by sidepseudo-opcodes. Each pseudo-opcode is further sub-divided into twovectors (a type of mini-opcode). Both vectors have access to the samefour data busses {Aop(31:0), Bop(31:0), Disp(31:0), and Const(3:0)} butuse the data differently depending on which reservation station protocolthat the reservation station entry has. There are a total of threereservation station entries. Every pseudo-opcode is one dispatchposition that carries an identification or DTAG denoted by the FIROBline number and entry number that will become its destination. Only theFIROB line number needs to be carried around since the entry number willcorrespond to a fixed dispatch position from 0 to 3. DTAG (destinationtags) can only be 0₋₋ 0 to 5₋₋ 3 which represents four dispatchpositions and six FIROB lines.

Allocation and Dispatch

Once the opcode has been dispatched to a reservation station (station 0,1, or 2), the station then watches the result bus tags for the remainingoperands that will be forwarded to it. Issue from the reservationstations to the functional units can be done out of order on Processor500. The oldest opcode (in program order) which has all of its operandswill be issued to the functional unit and the functional unit cannotstall. The decoder/dispatcher communicates with the FPU, LSSEC, and SRB,and FIROB and will stall if a new opcode(ready to be dispatched) has noplace to go. As an example, when all six FIROB lines are allocated orwhen all eight LSSEC entries are allocated, the dispatcher must stall.

Opcodes, tags, and operands for the reservation stations are alwaysdispatched into RS2. Issue to the functional unit can then come fromRS2, RS1, or RS0. When a another opcode is dispatched into RS2, theprevious opcode in RS2 is shifted down to RS1. If an opcode was in RS1,it would in turn be shifted to RS0.

Reservation Station Sub-Blocks

The reservation station logic (per dispatch/issue position) is dividedinto a control sub-block named RSCTL and three reservation stationsnamed RS2, RS1, and RS0. Opcodes, tags, and operands are only dispatchedto RS2, while any of the reservation stations can issue to the FNCU. Ifan operation gets all of its operands from the REGF and FIROB datasection and has no higher priority requests in the reservation stationsbefore it, it can go from the operand steering section directly to theFNCU for evaluation; otherwise, the operation waits in the reservationstation until its operands are forwarded to it. The entry being issuedto the FNCU is driven on a set of tristate busses. Another set oftristate busses is used by the Multiplier for source operands. Thesetristate busses (SAOPND and SBOPND) are shared by all four reservationblock positions, and a multiplier request/grant protocol determineswhich position drives them.

The RSCTL decides which operations end up in which reservation stations.A set of operands will stay in their current reservation station entryunless they are shifted to the next entry or sent to the FNCU forevaluation. No attempt is made to keep all operations pushed up againstone side or the other of the reservation station chain. A block diagramof reservation station logic is shown as FIG. 29. A timing diagram forthe reservation stations is shown as FIG. 30.

Reservation Station Protocols

Various protocols are used to indicate how the reservation stationshould use the two vectors stored in the VCBS (vector control bit store)section of the FNCU and which operands are needed.

Protocol Definitions

    ______________________________________                                        OP - operation only valid                                                      (v0: na {0}; v1: operation {F})                                              TOP - two opcode are fully supplied (not used by uCode)                       (fast path only)                                                               (v0: decode supplied; v1: decode supplied)                                   V12 - v0 fwd to v1                                                             (v0: Bop+const {I}; v1: operation {F)})                                      LO - Load-Operation                                                            (v0: Bop+disp {Lw}; v1: operation {F})                                       after v0, BTAG=DTAG                                                           LOS - Load-OP-Store                                                            (v0: Bop+disp {Lw}; v1: op & store {M})                                        after v0, BTAG=DTAG                                                         OS - op-store                                                                  (v0: Aop+disp {L}; v1: op & store {M})                                       SRB - SRB info                                                                 (v0: na {0}; v1:pass Bop {S})                                                FLA - fpu linear adr                                                           (v0: Bop+disp {I}; v1: fwd+const {L})                                         unlike regular linear adr calc; this one immediately                          clears the resv.sub.-- sta entry                                             BRN - Branch eval                                                              (v0: na {0}; v1: operation,brn eval {B})                                     LBR - Load-brn (RET)                                                           (v0: Aop+disp {Lw}; v1: operation,brn eval {B})                              SIB - 1st pos SIB                                                              (v0: Aop+scaled(Bop) {F}; v1: na {0})                                        PU - Push                                                                      (v0: Aop-const {L,F} v1: op & store {M})                                     POP - Pop                                                                      (v0: Aop+const {F} v1: na {0})                                               DIV - vector0 or vector1                                                       (v0: ADD {F}; v1: SUB {F})                                                   ______________________________________                                    

The following letters indicate which blocks should latch data off theRES bus (enclosed in {})

0 (nothing)

F (FIROB or FNCU forwarding)

L (linear address for store)

Lw (linear address for load; Res₋₋ sta changes Bop=DTAG and waits)

M (memory data for store only)

S (SRB)

B (Branch eval info for FIROB)

I (intermediate value; ignored for forwarding except for same Res sta)

Summary of How Protocols Apply to Various Instructions

    ______________________________________                                                  dispatch positions                                                            1st pos                                                                             2nd pos                                                       ______________________________________                                        regular operation                                                                         OP      / -- ;                                                    regular op w/ imm                                                                         OP      / -- ;                                                    Load-op     LO      / -- ;                                                    OP-store    OS      / -- ;                                                    Load-op-store                                                                             LOS     / -- ;                                                    SIB w/ Load-op                                                                            SIB     / LO ;                                                    SIB w/ OP-store                                                                           SIB     / OS ;                                                    SIB w/ L-O-S                                                                              SIB     / LOS ;                                                   divide opcode                                                                             DIV     / -- ;                                                    FPU linear adr                                                                            FLA     / -- ;                                                    Jcc         BRN     / -- ;                                                    JMP nr disp rel                                                                           BRN     / -- ;                                                    JMP nr reg indr                                                                           BRN     / -- ;                                                    JMP nr mem indr                                                                           LBR     / -- ;                                                    PUSH reg    PU      / -- ;                                                    PUSH mem    LO      / PU ;                                                    PUSH mem w/SIB                                                                            uCode                                                             PUSHF       PU      / -- ;   using MOVF                                       PUSHF w/OF fwd                                                                            V2      / PU ;   using MOVF and MOVOF                             CALL nr disp rel                                                                          PU      / BRN ;                                                   CALL nr reg indr                                                                          PU      / BRN ;  pos 2 adds indr.sub.-- reg w/                                                 zero instead of EIP+rel                          CALL nr mem indr                                                                          PU      / LBR ;                                                   POP mem     uCode                                                             POP mem w/SIB                                                                             uCode                                                             POPF        uCode   ;        this goes to uCode since                         IOPL can change                                                               RET         LBR     / OP ;                                                    RET imm     LBR     / V12 ;                                                   XCHG        uCode                                                             XADD        uCode                                                             LEAVE       POP     / LO ;                                                    LOOP        OP      / BRN ;                                                   LOOPcond    uCode                                                             JCXZ        OP      / BRN ;                                                   MUL 1 disp pos                                                                            OP      / -- ;                                                    MUL 2 disp pos                                                                            OP      / OP ;   2nd pos is a NOP                                 ______________________________________                                    

Operand to bus assignments and opcode details:

Aop,Bop, and Disp busses are 32 bits

Const bus is 4 bits

    __________________________________________________________________________           Aop  Bop  Disp                                                                              Const                                                                             operation                                                                             Latch                                        __________________________________________________________________________    regular operation                                                              ;            OP  /                                                           dsptch pos 1:                                                                 inputs→                                                                       A    B    --  --                                                       v0:                      none    {0}                                          v1:    A    B            operation                                                                             {F}                                          same but with immediate data                                                  regular operation                                                              ; OP.sub.-- STR block moves imm                                              to Bop                                                                        dsptch pos 1:                                                                 inputs→                                                                       A    imm  --  --                                                       v0:                      none    {0}                                          v1:    A    imm          operation                                                                             {F}                                           ;ad-op       LO  /                                                           dsptch pos 1:                                                                 inputs→                                                                            B    disp                                                                              --                                                       v0:         B    disp    Bop+disp                                                                              {Lw}                                                                  & Btag<=Dtag                                         v1:         LSRES        operation                                                                             {F}                                           ;-store      OS  /                                                           dsptch pos 1:                                                                 inputs→                                                                       A    B    disp                                                                              --                                                       v0:    A         disp    Aop+disp                                                                              {L}                                          v1:         B            operation                                                                             {M}                                           ;ad-op-store LOS /                                                           dsptch pos 1:                                                                 inputs→                                                                       A    B    --  --                                                       v0:    A         disp    Aop+disp                                                                              {Lw}                                                                  & Atag<=Dtag                                         v1:    LSRES                                                                              B            operation                                                                             {M}                                          SIB w/ Load-op                                                                              SIB / LO ;                                                      dsptch pos 1:                                                                 inputs→                                                                       base index                                                                              --  --                                                       v0:    base index        base+scaled(ind)                                                                      {F}                                          v1:                      none    {0}                                          dsptch pos 2:                                                                 inputs→                                                                            B    disp                                                                              --                                                       v0:         fwd  disp    fwd+disp                                                                              {Lw}                                                                  & Btag<=Dtag                                         v1:         LSRES        operation                                                                             {F}                                          SIB w/ OP-store                                                                             SIB / OS ;                                                      dsptch pos 1:                                                                 inputs→                                                                       base index                                                                              --  --                                                       v0:    base index        base+scaled(ind)                                                                      {F}                                          v1:                      none    {0}                                          dsptch pos 2:                                                                 inputs→                                                                       A    B    disp                                                                              --                                                       v0:    fwd       disp    fwd+disp                                                                              {L}                                          v1:         B            operation                                                                             {M}                                          SIB w/ L-O-S  SIB / LOS ;                                                     dsptch pos 1:                                                                 inputs→                                                                       base index                                                                              --  --                                                       v0:    base index        base+scaled (ind)                                                                     {F}                                          v1:                      none    {0}                                          dsptch pos 2:                                                                 inputs→                                                                       A    B    disp                                                                              --                                                       v0:    fwd       disp    fwd+disp                                                                              {Lw}                                                                  & Atag<=Dtag                                         v1:    LSRES                                                                              B            operation                                                                             {M}                                           ;vide opcode DIV /                                                           dsptch pos 1:                                                                 inputs→                                                                       A    B    --  --                                                       v0:    A    B            ADD A,B {F}                                          v1:    A    B            SUB A,B {F}                                          FPU linear address                                                             ;            FLA /                                                           dsptch pos 1:                                                                 inputs→                                                                            B    disp                                                                              const                                                    v0:         B    disp    Bop+disp                                                                              {I}                                                                   & Btag<=Dtag                                         v1:         fwd      const                                                                             fwd+const                                                                             {L}                                           ;c           BRN /                                                           dsptch pos 1:                                                                 inputs→                                                                       EIP  pre.sub.-- adr                                                                     rel --                                                       v0:                      none    {0}                                          v1:    EIP  B    rel     EIP+rel {B}                                          next cycle flag logic compares EIP+rel & pre.sub.-- adr for                   predicted taken                                                               JMP nr disp rel                                                                ; same as conditional                                                        except always taken                                                           dsptch pos 1:                                                                 inputs→                                                                       EIP  pre.sub.-- adr                                                                     rel --                                                       V0:                      none    {0}                                          v1:    EIP  B    rel     EIP+rel {B}                                          next cycle flag logic compares EIP+rel & pre.sub.-- adr for                   predicted taken                                                               JMP nr reg indr                                                                ;            BRN /                                                           dsptch pos 1:                                                                 inputs→                                                                       reg  pre.sub.-- adr                                                                     --  0                                                        v0:                      none    {0}                                          v1:    reg           0   reg+0   {B}                                          next cycle flag logic compares reg+0 & pre.sub.-- adr for                     predicted taken                                                               JMP nr mem indr                                                                ;            LBR /                                                           dsptch pos 1:                                                                 inputs→                                                                       Aop  pre.sub.-- adr                                                                     disp                                                                              0                                                        v0:    Aop       disp    Aop+disp                                                                              {Lw}                                                                  & Atag<=Dtag                                         v1:    LSRES         0   LSRES+0 {B}                                          next cyclee flag logic compares LSRES+0 & pre.sub.-- adr for                  predicted taken                                                                ;SH reg      PU  /                                                           dsptch pos 1:                                                                 inputs→                                                                       ESP  B    --  const                                                    v0:    ESP           const                                                                             ESP-const                                                                             {L,F}                                        v1:         B            operation                                                                             {M}                                           ;SH mem      LO  /                                                           dsptch pos 1:                                                                 inputs→                                                                            B    disp                                                                              --                                                       v0:         B    disp    Bop+disp                                                                              {Lw}                                                                  & Btag<=Dtag                                         v1:         LSRES        operation                                                                             {F}                                          dsptch pos 2:                                                                 inputs→                                                                       ESP  fwd  --  --                                                       v0:    ESP           const                                                                             ESP-const                                                                             {L,F}                                        v1:         fwd          operation                                                                             {M}                                           ; using MOVF PU  /                                                           dsptch pos 1:                                                                 inputs→                                                                       ESP  flg  --  const                                                    v0:    ESP           const                                                                             ESP-const                                                                             {L,F}                                        v1:         flg          MOVF    {M}                                          the MOVF on v1 combines system flags on Bop with CF & XF                      PUSH w/OF fwd OP  / PU ; using MOVF and MOVOF                                 dsptch pos 1:                                                                 inputs→                                                                       --   flg  --  --                                                       v0:                      none    {0}                                          v1:         flg          MOVF    {F}                                          the MOVF on v1 combines system flags on Bop with CF & XF                      dsptch pos 2:                                                                 inputs→                                                                       ESP  fwd  --  const                                                    v0:    ESP           const                                                                             ESP-const                                                                             {L,F}                                        v1:         fwd          MOVOF   {M}                                          the MOVOF on v1 overwrites the OF bit position of Bop                         Call nr disp rel                                                                            PU  / BRN ;                                                     dsptch pos 1:                                                                 inputs→                                                                       ESP  EIP  --  const                                                    v0:    ESP           const                                                                             ESP-const                                                                             {L,F}                                        v1:         EIP          operation                                                                             {M}                                          dsptch pos 2:                                                                 inputs→                                                                       EIP  pre-adr                                                                            disp                                                                              --                                                       v0:                      none    {0}                                          v1:    EIP       disp                                                                              --  EIP+disp                                                                              {B}                                          next cycle flag logic compares EIP+disp & pre.sub.-- adr for                  predicted taken                                                               CALL nr reg indr                                                                            PU  / BRN ; pos 2 adds indr.sub.-- reg w/                       zero instead of EIP+rel                                                       dsptch pos 1:                                                                 inputs→                                                                       ESP  EIP  --  const                                                    v0:    ESP           const                                                                             ESP-const                                                                             {L,F}                                        v1:         EIP          operation                                                                             {M}                                          dsptch pos 2:                                                                 inputs→                                                                       reg  pre.sub.-- adr                                                                     0   --                                                       v0:                      none    {0}                                          v1:    reg       0   --  reg+0   {B}                                          next cycle flag logic compares reg+0 & pre.sub.-- adr for                     predicted taken                                                               CALL nr mem indr                                                                            PU  / LBR ;                                                     dsptch pos 1:                                                                 inputs→                                                                       ESP  EIP  --  const                                                    v0:    ESP           const                                                                             ESP-const                                                                             {L,F}                                        v1:         EIP          operation                                                                             {M}                                          dsptch pos 2:                                                                 inputs→                                                                       A    pre.sub.-- adr                                                                     disp                                                                              --                                                       v0:    A         disp    Aop+disp                                                                              {Lw}                                                                  & Atag<=Dtag                                         v1:    LSRES             LSRES+0 {B}                                          next cycle flag logic compares LSRES+0 & pre.sub.-- adr for                   predicted taken                                                               POP reg       LO  / POP ;                                                     dsptch pos 1:                                                                 inputs→                                                                            ESP  0   --                                                       v0:         ESP  0       ESP+0   {Lw}                                                                  & Btag<=Dtag                                         v1:         LSRES        operation                                                                             {F}                                          dsptch pos 2:                                                                 inputs→                                                                       ESP  --   --  const                                                    v0:    ESP           const                                                                             ESP+const                                                                             {F}                                          v1:                      none    {0}                                          RET           LBR / OP ;                                                      dsptch pos 1:                                                                 inputs→                                                                       ESP  pre.sub.-- adr                                                                     --  0                                                        v0:    ESP           0   ESP+0   {Lw}                                                                  & Atag<=Dtag                                         v1:    LSRES         0   LSRES+0 {B}                                          next cycle flag logic compares LSRES+0 & pre.sub.-- adr for                   predicted taken                                                               dsptch pos 2:                                                                 inputs→                                                                       ESP  --   --  const                                                    v0:    ESP           const                                                                             ESP+const                                                                             {F}                                          v1:                      none    {0}                                          RET imm       LBR / V12 ;                                                     dsptch pos 1:                                                                 inputs→                                                                       ESP  pre.sub.-- adr                                                                     0   --                                                       v0:    ESP       0       ESP+0   {Lw}                                                                  & Atag<=Dtag                                         v1:    LSRES     0       LSRES+0 {B}                                          next cycle flag logic compares LSRES+0 & pre.sub.-- adr for                   predicted taken                                                               dsptch pos 2:                                                                 inputs→                                                                       ESP  imm      const                                                    v0:    ESP           const                                                                             ESP+const                                                                             {I}                                          v1:    fwd  imm          fwd+imm {F}                                          LEAVE         V12 / LO ;                                                      dsptch pos 1:                                                                 inputs→                                                                            EBP  --  const                                                    v0:                  const                                                                             EBP+const                                                                             {I}                                          v1:         fwd          R<-fwd  {F}                                          this one is used so another protocol is not needed and                        dependency checking is easy; otherwise, use a version of the                  POP protocol with Bop+const.                                                  FIROB latches the output as the new ESP value.                                dsptch pos 2:                                                                 inputs→                                                                            EBP  0   --                                                       v0:         EBP  0       EBP+0   {Lw}                                                                  & Btag<=Dtag                                         v1:         LSRES        operation                                                                             {F}                                          FIROB latches v1 result as the new EBP value                                  LOOP          OP  / BRN ;                                                     dsptch pos 1:                                                                 inputs→                                                                       ECX  1    --  --                                                       v0:                      none    {0}                                          v1:    ECX  1            operation                                                                             {F}                                          dsptch pos 2:                                                                 inputs→                                                                       EIP  pre.sub.-- adr                                                                     disp                                                                              --                                                       v0:                      none    {0}                                          v1:    EIP       disp                                                                              --  EIP+disp                                                                              {B}                                          next cycle flag logic compares EIP+disp & pre.sub.-- adr for                  predicted taken                                                               JCXZ          OP  / BRN ;                                                     dsptch pos 1:                                                                 inputs→                                                                       ECX  0    --  --                                                       v0:                      none    {0}                                          v1:    ECX  0            operation                                                                             {F}                                          dsptch pos 2:                                                                 inputs→                                                                       EIP  pre.sub.-- adr                                                                     disp                                                                              --                                                       v0:                      none    {0}                                          v1:    EIP       disp                                                                              --  EIP+disp                                                                              {B}                                          next cycle flag logic compares EIP+disp & pre.sub.-- adr for                  predicted taken                                                               MUL 1 disp pos                                                                 ;            OP  /                                                           dsptch pos 1:                                                                 inputs→                                                                       A    B    --  --                                                       v0:                      none    {0}                                          resv.sub.-- sta receives grant and sends source on SAOPND &                   SBOPND; FNCU is idle                                                          resv.sub.-- sta starts counting 3 cycles and issures v1 to pass               MUL result                                                                    v1:                      pass MUL                                                                              {F}                                          MUL 2 disp pos                                                                              OP  / OP ; 2nd pos is a NOP                                     dsptch pos 1:                                                                 inputs→                                                                       A    B    --  --                                                       v0:                      none    {0}                                          resv.sub.-- sta receives grant and sends source on SAOPND &                   SBOPND; FNCU is idle                                                          resv.sub.-- sta starts counting 3 cycles and issues v1 to pass                MUL result                                                                    resv.sub.-- sta(i) signals resv.sub.-- sta(i+1) to start counting 4           cycles                                                                        v1:                      pass MUL                                                                              {F}                                          dsptch pos 2:                                                                 inputs                                                                        v0:                      NOP     {0}                                          resv.sub.-- sta starts counting 4 cycles and issues v1 to pass                MUL result                                                                    v1:                      NO      {F}                                          __________________________________________________________________________

Cycle Type

Sometimes incorrect data will be sent by the Dcache or the LSSEC. Thereservation station will resend the cancelled cycle after the correctdata comes from the data cache in the case of a hit in a mispredictedway. When the DCUNPAHIT or DCUNPBHIT (Dcache hit in unpredicted way)signal is detected, the R₋₋ VALID status is changed to an R₋₋ NONEstatus by the FNCU and the reservation station will latch in the correctdata next cycle. The correct data from an unpredicted way will have towait one extra cycle to be issued since the reservation station hasalready prioritized another entry for issue. DCUNPAHIT and DCUNPBHITcorrespond to Dcache results for the data on LSRES0 and LSRES1respectively. A Dcache miss is detected when valid tags match on LSTAG0bus but no DCUNPAHIT or DCPRPAHIT is detected. The DCUNPBHIT andDCPRPBHIT signals are for the LSTAG1 bus status. The LSSEC uses asimilar protocol for miss only by sending the LSCANFWD signal whichindicates that the forwarded data was incorrect. When load data isreceived on the LSRES bus, the reservation station keeps thecorresponding entry for a cycle after it is issued to the FNCU andchecks to see if its tags needed to be set again for the miss condition.Keeping the reservation station entry valid for an extra cycle is notneeded in the case of an FPU load linear address calculation. The FPUreservation station must be responsible for detecting and handling theincorrect load data conditions. Another side note is that AHBYTE data isnot sent back to bits 15:8 for "store" data.

SRB, FPU and MULT Handling

SRB opcodes look like a move instruction with the exception that aSTAT₋₋ SRB signal is asserted to let the LSSEC know the 32 bit valueneeds to be latched into the 4 entry SRB file. FPU opcodes go to uCodeand get dispatched on a single FIROB line which uses the first threepositions for calculating linear addresses and the last position for theentry point into the Fcode (FPU microcode). FPU entry points are sentdirectly from dispatch to the FPU.

Each of the four dispatch positions can send a multiply request signalto the multiplier unit, and wait to see which position receives a grantsignal. Once a reservation station position has the grant it can use theSAOPND and SBOPND busses to send its sources to the multiplier. It alsostarts counting three cycles. Nothing except a DTAG, valid status, and aselection signal are sent to the FNCU at the end of the count. Themultiply result will be multiplexed onto the FNCU's RES bus. If tworesults are going to be written by the multiplier, the reservationstation that gets the multiplier grant will send a count start signal tothe (I+1) reservation station position. After four cycles the (I+1)reservation station will finish its count and let the second multiplyresult pass onto the RES bus for it's FNCU.

Reservation Station Signals

This signal list covers the reservation station at the top of itshierarchy. There are three reservation station entries covered in the RSsignal list and a control section signal list covered in the RSCTL.

Reservation Station Input Signals

AXBLAC3--use A operand or B operand for linear address calc

DCPAPRHIT--data cache port A predicted way hit

DCPBPRHIT--data cache port B predicted way hit

DCUNPAHIT--data cache port A unpredicted way hit

DCUNPBHIT--data cache port B unpredicted way hit

FOCTLST₋₋ VUSE3--valid control store use next cycle

FOCTLST₋₋ VWR3--valid control store write next cycle

MULCYC2X3₋₋ 3--indicates 2 or 3 cycle multiply is requiredMULGRn--multiply grant to position n1

MULRES3₋₋ 3--leave RES bus free for MUL result in 3 cycles MULX;

RD₋₋ DEP₋₋ DS3--entry specific read bit for DS; entry wait if both readand write are set

RD₋₋ DEP₋₋ ES3--entry specific read bit for ES; entry wait if both readand write are set

RD₋₋ DEP₋₋ FS3--entry specific read bit for FS; entry wait if both readand write are set

RD₋₋ DEP₋₋ GS3--entry specific read bit for GS; entry wait if both readand write are set

RQLAC3--request linear address calc

VAT3--valid A operand tag

VBT3--valid B operand tag

VCFT3--valid carry flag operand tag

VRS3--valid entry going from op steer to RS2

VSFT3--valid status flag operand tag

WRDS--global bit for write to DS

WRES--global bit for write to ES

WRFS--global bit for write to FS

WRGS--global bit for write to GS

10:0! INSSEG--LSSEC segment MUX select

10:0! RSPROTOCOL3--determines protocol (ie: LOS, MUL, DIV, etc.)

1:0! LSCANFWD--LSSEC cancel signal for forwarded data on the LSRESn bus

2:0! CDTAG3--current destination tag for the incoming opcode for RS2

31:0! AOPND3--A operand

31:0! BOPND3--B operand

31:0! DSPR3--displacement

31:0! LSRES0--LSSEC result bus 0

31:0! LSRES1--LSSEC result bus 1

31:0! RES0--forwarded result bus from position 0

31:0! RES1--forwarded result bus from position 1

31:0! RES2--forwarded result bus from position 2

31:0! RES3--forwarded result bus from position 3

3:0! FOCTLST₋₋ WR--indicates which control store vector to use

4:0! ATAG3--A operand tag sent to RS2

4:0! BTAG3--B operand tag sent to RS2

4:0! CFTAG3--carry (CF) tag sent to RS2

4:0! DTAG0--destination ID for result at position 0

4:0! DTAG1--destination ID for result at position 1

4:0! DTAG2--destination ID for result at position 2

4:0! DTAG3--destination ID for result at position 3

4:0! LSTAG0--destination ID for LS result at position 0

4:0! LSTAG1--destination ID for LS result at position 1

4:0! SFTAG3--status flags (ZF,AF,PF,SF,DF,OF) tag sent to RS2

6:0! FLGD3--input flag data from the reservation station

6:0! RFLAG0--input flag data from position 0

6:0! RFLAG1--input flag data from position 1

6:0! RFLAG2--input flag data from position 2

6:0! RFLAG3--input flag data from position 3

6:0! RFLAGLS0--input flag data from LSSEC 0

6:0! RFLAGLS1--input flag data from LSSEC 1

7:0! FOCTLST₋₋ USE3--indicates which control store vector to use

Reservation Station Output Signal List

F0CTLST₋₋ VUSE--valid control store use next cycle

F0CTLST₋₋ VWR--valid control store write next cycle

FNCUGO--functional unit GO indication

FUGNT3--indicates for operand steer to drive the shared data busses intothe FNCU

MULCYC2X3--number of multiply cycles is 2, else use 3

MULONEOP--opcode is for the one operand version

MULRES3--leave RES bus free for MUL result in 3 cycles

MULRQn--multiply request from position n

MULSIGN--signed values

OPALSRES0--used to qualify valid status; else mispredicted way miss; Aop& DC port 0

OPALSRES1--used to qualify valid status; else mispredicted way miss; Aop& DC port 1

OPBLSRES0--used to qualify valid status; else mispredicted way miss; Bop& DC port 0

OPBLSRES1--used to qualify valid status; else mispredicted way miss; Bop& DC port 1

RSFULL--reservation station full

10:0! INSSEG--LSSEC segment MUX select

1:0! MULOPSIZE--output operands are 01: byte, 10: word, or 11: dword

2:0! CDTAG--current destination tag for the incoming opcode YY use DTAG

31:0! AOPND--A operand

31:0! BOPND--B operand

31:0! DSPR--displacement

31:0! SAOPND--shared A operand bus

31:0! SBOPND--shared B operand bus

3:0! F0CTLST₋₋ WR--indicates which control store vector to use

6:0! FLGD--input flag data from the reservation station

6:0! FNCUMUXA--mux control from reservation station for A operand input

6:0! FNCUMUXB--mux control from reservation station for B operand input

6:0! FNCUMUXCF--mux control from reservation station for the carry flag

6:0! FNCUMUXSF--mux control from reservation station for the statusflags

7:0! FOCTLST₋₋ USE--indicates which control store vector to use

RS: Reservation Station Entry Signal List

Three identical reservation station entries for each reservation stationblock allow storage and forwarding for three outstanding dispatchpositions.

RS Input Signal List

AXBLACI--use A operand or B operand for linear address calc

FOCTLST₋₋ VUSEI--valid control store use next cycle

FOCTLST₋₋ VWRI--valid control store write next cycle

MULCYC2X3I--2 or 3 cycle multiply

MULGRI--multiply grant to position n1

MULRES3I--leave RES bus free for MUL result in 3 cycles

RD₋₋ DEP₋₋ DSI--entry specific read bit for DS; entry wait if both readand write are set

RD₋₋ DEP₋₋ ESI--entry specific read bit for ES; entry wait if both readand write are set

RD₋₋ DEP₋₋ FSI--entry specific read bit for FS; entry wait if both readand write are set

RD₋₋ DEP₋₋ GSI--entry specific read bit for GS; entry wait if both readand write are set

RQLACI--request linear address calc

VATI--valid A operand tag

VBTI--valid B operand tag

VCFTI--valid carry flag operand tag

VRSI--valid entry going from op steer to RS2

VSFTI--valid status flag operand tag

10:0! INSSEGI--LSSEC segment MUX select

10:0! RSPROTOCOLI--determines protocol (ie: LOS, MUL, DIV, etc.)

2:0! CDTAGI--current destination tag for the incoming opcode for RS2

31:0! AOPNDI--A operand

31:0! BOPNDI--B operand

31:0! DSPRI--displacement

31:0! LSRES0--LSSEC result bus 0

31:0! LSRES1--LSSEC result bus 1

31:0! RES0--forwarded result bus from position 0

31:0! RES1--forwarded result bus from position 1

31:0! RES2--forwarded result bus from position 2

31:0! RES3--forwarded result bus from position 3

3:0! FOCTLST₋₋ WRI--indicates which control store vector to use

4:0! ATAGI--A operand tag sent to RS2

4:0! BTAGI--B operand tag sent to RS2

4:0! CFTAGI--carry (CF) tag sent to RS2

4:0! DTAG0--destination ID for result at position 0

4:0! DTAG1--destination ID for result at position 1

4:0! DTAG2--destination ID for result at position 2

4:0! DTAG3--destination ID for result at position 3

4:0! LSTAG0--destination ID for LS result at position 0

4:0! LSTAG1--destination ID for LS result at position 1

4:0! SFTAGI--status flags (ZF,AF,PF,SF,DF,OF) tag sent to RS2

6:0! FLGDI--input flag data from the reservation station

6:0! RFLAG0--input flag data from position 0

6:0! RFLAG1--input flag data from position 1

6:0! RFLAG2--input flag data from position 2

6:0! RFLAG3--input flag data from position 3

6:0! RFLAGLS0--input flag data from LSSEC 0

6:0! RFLAGLS1--input flag data from LSSEC 1

7:0! FOCTLST₋₋ USEI--indicates which control store vector to use

RS Output Signal List

AXBLACO--use A operand or B operand for linear address calc

F0CTLST₋₋ VUSE--valid control store use next cycle

F0CTLST₋₋ VUSEO valid control store use next cycle

F0CTLST₋₋ VWR--valid control store write next cycle

F0CTLST₋₋ VWRO--valid control store write next cycle

FNCUGO--functional unit GO indication

MULCYC2X3--number of multiply cycles is 2, else use 3

MULCYC2X3O

MULGRO--multiply grant to position n1

MULONEOP--opcode is for the one operand version

MULRES3--leave RES bus free for MUL result in 3 cycles

MULRES30--leave RES bus free for MUL result in 3 cycles

MULSIGN--signed values

OPALSRES0--used to qualify valid status; else mispredicted way miss; Aop& DC port 0

OPALSRES1--used to qualify valid status; else mispredicted way miss; Aop& DC port 1

OPBLSRES0--used to qualify valid status; else mispredicted way miss; Bop& DC port 0

OPBLSRES1--used to qualify valid status; else mispredicted way miss; Bop& DC port 1

RD₋₋ DEP₋₋ DSO--entry specific read bit for DS; entry wait if both readand write are set

RD₋₋ DEP₋₋ ESO--entry specific read bit for ES; entry wait if both readand write are set

RD₋₋ DEP₋₋ FSO--entry specific read bit for FS; entry wait if both readand write are set

RD₋₋ DEP₋₋ GSO--entry specific read bit for GS; entry wait if both readand write are set

RQLACO--request linear address calc

VATO--valid A operand tag

VBTO--valid B operand tag

VCFTO--valid carry flag operand tag

VRSO--valid entry going from op steer to RS2

VSFTO--valid status flag operand tag

10:0! INSSEG--LSSEC segment MUX select

10:0! INSSEGO--LSSEC segment MUX select

10:0! RSPROTOCOLO--determines protocol (ie: LOS, MUL, DIV, etc.)

1:0! MULOPSIZE--output operands are 01: byte, 10: word, or 11: dword

2:0! CDTAG--current destination tag for the incoming opcode YY use DTAG

2:0! CDTAGO--current destination tag for the incoming opcode for RS2

31:0! AOPND--A operand

31:0! AOPNDO--A operand

31:0! BOPND--B operand

31:0! BOPNDO--B operand

31:0! DSPR--displacement

31:0! DSPRO--displacement

31:0! SAOPND--shared A operand bus

31:0! SBOPND--shared B operand bus

3:0! FOCTLST₋₋ WR--indicates which control store vector to use

3:0! FOCTLST₋₋ WRO--indicates which control store vector to use

4:0! ATAGO--A operand tag sent to RS2

4:0! BTAGO--B operand tag sent to RS2

4:0! CFTAGO--carry (CF) tag sent to RS2

4:0! SFTAGO--status flags (ZF,AF,PF,SF,DF,OF) tag sent to RS2

6:0! FLGD--input flag data from the reservation station

6:0! FLGDO--input flag data from the reservation station

6:0! FNCUMUXA--mux control from reservation station for A operand input

6:0! FNCUMUXB--mux control from reservation station for B operand input

6:0! FNCUMUXCF--mux control from reservation station for the carry flag

6:0! FNCUMUXSF--mux control from reservation station for the statusflags

7:0! FOCTLST₋₋ USE--indicates which control store vector to use

7:0! FOCTLST₋₋ USEO--indicates which control store vector to use

RSCTL Sub-Block

Controls shifting of entries within the reservation station and controlswhat the FNCU does and which control bit vector is used.

RSCTL Input Signal List

AXBLACx--use A operand or B operand for linear address calc

DCPAPRHIT--data cache port A predicted way hit

DCPBPRHIT--data cache port B predicted way hit

DCUNPAHIT--data cache port A unpredicted way hit

DCUNPBHIT--data cache port B unpredicted way hit

MATx--match on A operand tag

MBTx--match on B operand tag

MCFTx--match on carry flag (CF) tag

MSFTx--match on status flag (STF) tag

MULCYC2X3₋₋ x--multiply takes 2 or 3 cycles

MULGRn;

MULRES3₋₋ x--leave RES bus free for MUL result in 3 cycles

RD₋₋ DEP₋₋ DSx--entry specific read bit for DS; entry wait if both readand write are set

RD₋₋ DEP₋₋ ESx--entry specific read bit for ES; entry wait if both readand write are set

RD₋₋ DEP₋₋ FSx--entry specific read bit for FS; entry wait if both readand write are set

RD₋₋ DEP₋₋ GSx--entry specific read bit for GS; entry wait if both readand write are set

RQLACX--request linear address calc

VATX--valid A operand tag

VBTx--valid B operand tag

VCFTx--valid carry flag operand tag

VRSx--valid entry going from op steer to RS2

VSFTx--valid status flag operand tag

WRDS--global bit for write to DS

WRES--global bit for write to ES

WRFS--global bit for write to FS

WRGS--global bit for write to GS

10:0! RSPROTOCOLx--determines protocol (ie: LOS, MUL, DIV, etc.)

1:0! LSCANFWD--LSSEC cancel signal for forwarded data on the LSRESn bus

RSCTL Output Signal List

FNCUGO--functional unit GO indication

FUGNT0--indicates for operand steer to drive the shared data busses intothe FNCU

FUGNT1--indicates for operand steer to drive the shared data busses intothe FNCU

FUGNT2--indicates for operand steer to drive the shared data busses intothe FNCU

FUGNT3--indicates for operand steer to drive the shared data busses intothe FNCU

MULRQn--request use of multiplier

R0SHF--shift new data into entry 0

R1SHF--shift new data into entry 1

R2SHF--shift new data into entry 2

RSFULL--reservation station full

Operand Steering

The operand steering sections purpose is to give an extra cycle togather source data together for the reservation station or functionalunit and to provide time to decode opcodes into two vectors of fullydecoded control signals for the functional unit. Since several units maybe sending data/tags to the operand steering block at the same time, theoperand steering section uses the following priority: 1) tags, 2) FIROBdata, 3) REGF. The incoming data for the two sources may also be in theincorrect position, and the operand steering section will need to swapthe sources to the correct positions for the A and B operands. Theoperand steering section will also need to detect any forwarding andhave RS2 latch in the forwarded data. The operand steering sectioncannot issue directly to the functional unit while using forwarded data.Reservation station entries however can be issued to the functional unitwhile forwarded busses arrive at the FNCU inputs at the same cycle.

Tag Bus

The ATAG and BTAG busses are used for renaming when the actual operandvalue is not available to be sent to the reservation station. The tagtakes the form of three bits representing the FIROB line. The entryposition on the FIROB line is understood by the physical placement ofeach operand steering section. Since each functional unit only drivesits own dedicated result bus, the reservation stations compare for theFIROB line value on the three bits of the corresponding result bus Thetag entry on a given FIROB line is known by the physical placement ofthe FNCU and its dedicated result bus. The only exception is data forthe LSRES1 and LSRESO busses; the tag of the entry that made the loadrequest is driven on the LSTAG0 or LSTAG1 busses. For example when theoperation for FIROB line 2 entry 3 is waiting on the LSSEC for the loaddata, it will compare both the LSRESO and LSRES1 busses for the 010₋₋ 11tag. Instead of watching for a tag from another FNCU, the reservationstation is watching its own tag to be sent along with load data from theLSSEC.

Vector Generation for the Vector Bit Control Store (VBCS)

During the operand steering cycle, the two vectors per dispatch positionentry will be generated. The FNCU does not have time to decode anopcode, execute the opcode, and generate flags in one cycle. The opcodegeneration has been pushed back in to the operand steering section whilethe flag generation has been pushed out to the cycle after execution. Aseven bit opcode comes from decode to the operand steering section alongwith five bits of reservation station control protocol. This informationand some other signals such as size, ahbyte, etc. are used by theoperand steering section to generate the two vectors. Sometimes thefirst vector will be used for linear address generation while the secondis used for the actual opcode. For the divide opcodes there will be adivision direction signal held locally at the FNCUO. This signaldetermines when to use vector0 (ADD) or vector1 (SUB).

Reservation Station Protocol Control

This table is used to give control signals to the reservation stationand decide what types of control vectors and status signals to generate.

    ______________________________________                                        OP - operation only valid                                                      (v0: na {0}; v1: operation {F})                                              TOP - two opcode are fully supplied (not used by uCode)                       (fast path only)                                                              (v0: decode supplied; v1: decode supplied)                                    V12 - v0 fwd to v1                                                             (v0: Bop+const {I}; v1: operation {F})                                       LO - Load-Operation                                                            (v0: Bop+disp {Lw}; v1: operation {F})                                         after v0, BTAG=DTAG                                                         LOS - Load-OP-Store                                                            (v0: Aop+disp {Lw}; v1: op & store {M})                                        after v0, BTAG=DTAG                                                         OS - op-store                                                                  (v0: Aop+disp {L}; v1: op & store {M})                                       SRB - SRB info                                                                 (v0: na {0}; v1:pass Bop {S})                                                FLA - fpu linear adr                                                           (v0: Bop+disp {I}; v1: fwd+const {L})                                         unlike regular linear adr calc; this one immediately                         clears the resv.sub.-- sta entry                                              BRN - Branch eval                                                              (v0: na {0}; v1: operation,brn eval {B})                                     LBR - Load-brn (RET)                                                           (v0: Aop+disp {Lw}; v1: operation,brn eval {B})                              SIB - 1st pos SIB                                                              (v0: Aop+scaled(Bop) {F}; v1: na {0})                                        PU - Push                                                                      (v0: Aop-const {L,F} v1: op & store {M})                                     POP - Pop                                                                      (v0: Aop+const {F} v1: na {0})                                               DIV - vector0 or vector1                                                       (v0: ADD {F}; v1: SUB {F})                                                   ______________________________________                                    

The following letters indicate which blocks should latch data off theRES bus (enclosed in {}):

0 (nothing)

F (FIROB or FNCU forwarding)

L (linear address for store)

Lw (linear address for load; Res₋₋ sta changes Bop=DTAG and waits)

M (memory data for store only)

S (SRB)

B (Branch eval info for FIROB)

I (intermediate value; ignored for forwarding except for same Res sta)

Definitions of the Flag Equations and the Opcode Bus

Flag Equation Groups

    ______________________________________                                        ADD.sub.-- F                                                                             3'b000   // flag equations for addition, sub,                                             etc.                                                   LOG.sub.-- F                                                                             3'b001   // flag equations for logical                             CNC.sub.-- F                                                                             3'b010   // flag equations for complement CF                       BCD.sub.-- FS                                                                            3'b011   // flag equations for AAA, AAS                            BCD.sub.-- FC                                                                            3'b100   // flag equations for AAA, AAS                            DAA.sub.-- FS1                                                                           3'b101   // flag equations for DAA, DAS                            STD.sub.-- F        // flag equations for STD                                 CLD.sub.-- F        // flag equations for CLD                                 ______________________________________                                    

The opcode groups form the first three bits of the opcode. A fourth bitmay be needed if new opcodes are added.

Opcode Groups

    ______________________________________                                        G.sub.-- ALU         3'b000  //                                               G.sub.-- BRN.sub.-- ALU                                                                            3'b001  //                                               G.sub.-- XFR         3'b010  //                                               G.sub.-- SFT         3'b011  //                                               G.sub.-- BIT         3'b100  //                                               G.sub.-- MUL         3'b101  //                                               G.sub.-- DIV         3'b110  //                                               G.sub.-- IDIV        3'b111  //                                               ______________________________________                                    

Basic ALU group

    ______________________________________                                        ADD      3'b000   //     R <- A + B  p=1001,                                                           g=110                                                OR       3'b001   //     R <- A + B  p=1000,                                                           g=111                                                ADC      3'b010   //     R <- A + B + CF                                                                           p=1001,                                                           g=110                                                SBB      3'b011   //     R <- A - B + CF                                                                           p=0110,                                                           g=101                                                AND      3'b100   //     R <- A & B  p=1110,                                                           g=111                                                SUB      3'b101   //     R <- A - B  p=0110,                                                           g=101                                                XOR      3'b110   //     R <- A   B  p=1001,                                                           g=111                                                MULH     3'b111   //     this opcode is a NOP which                                                    holds a res sta slot                                                          and waits for                                                          //     the 2nd multiply result to                                                    request to drive data to                                                      the FNCU                                             ______________________________________                                    

Branch and extended ALU Group

    ______________________________________                                        (for BRN >>> FNCU receives: A <- EIP, disp <- rel, B <- pred                  addr)                                                                         ______________________________________                                        JMPRI  3'b000  //    R <- A register indirect                                                      (uncond); source = r16,r32,  r16+d16!,                                        or  r32+d32! & set R.sub.-- BRN.sub.-- ADR               JMP    3'b001  //    R <- A + disp jump relative (uncond)                                          & set R.sub.-- BRN.sub.-- ADR                            JMPCC  3'b010  //    R <- A + disp jump relative if                                                condition true &                                                              set R.sub.-- BRN.sub.-- OK, R.sub.-- BRN.sub.-- NT,                           or                                                                            R.sub.-- BRN.sub.-- ADR                                  SETCC  3'b011  //    R(0) <- 1'b1 set result bit 0 if                                              condition true                                           ANDN   3'b100  //    R <- A & XB p=1101, g=111                                SUBR   3'b101  //    R <- B - A p=0110, g=011                                 DFADD  3'b110  //    R <- A + B p=1001, g=110 <-when                                               EFLAGDF=0                                                               //    R <- A - B p=0110, g=101 <-when                                               EFLAGDF=1                                                MOVCC  3'b111  //    R <- B p=1010, g=111 <-when                                                   condition is true                                                       //    R <- A p=1010, g=111                                                          ignore adder result and pass Aop when                                         cond. false                                              ______________________________________                                         // * note:                                                                    JMP w/ R <- immed will be handled in the IFETCH for absolute jump using       immediate value                                                          

Transfer Group

    ______________________________________                                        MOVSX     3'b000   //    R <- B  p=1010, g=111 sign                                                            extend B                                     MOVZX     3'b001   //    R <- B  p=1010, g=111 zero                                                            extend B                                     SIGNXA    3'b010   //    R <- A  p=1100, g=111 sign                                                            extend A                                     ZEROXA    3'b011   //    R <- A  p=1100, g=111 zero                                                            extend A                                     MOVDSPSX  3'b100   //    R <- disp                                                                             p=1010, g=111 sign                                                            extend disp                                  MOVDSPZX  3'b101   //    R <- disp                                                                             p=1010, g=111 zero                                                            extend disp                                  LEAA      3'b110   //    R <- A+disp p=1001, g=110 loads                                               effective address (Aside)                            LEAB      3'b111   //    R <- B+disp p=1001, g=110 loads                                               effective address (Bside)                            ______________________________________                                    

Shift Group

    ______________________________________                                        ROL      3'b000   //     rotate left                                          ROR      3'b001   //     rotate right                                         RCL.sub.-- 1                                                                           3'b010   //     rotate left through carry (1 bit                                              only)                                                RCR.sub.-- 1                                                                           3'b011   //     rotate right through carry (1 bit                                             only)                                                SHL      3'b100   //     shift left arithmetic or logical                     SHR      3'b101   //     shift right logical                                  RC0      3'b110   //     initial step of rotate through                                                carry sequence                                       SAR      3'b111   //     shift right arithmetic                               ______________________________________                                    

Bit group

    ______________________________________                                        BT         3'b000  //    bit test                                             LAHF       3'b001  //    load flags into AH                                                            R <- flags                                                                    (sf:zf:00:af:00:pf:00:df)                            SAHF       3'b010  //    store AH into flags R <- A;                                                   flags <- A(low byte)                                                          (sf:zf:00:af:00:pf:00:df)                            MOVF       //    combine Bop and flag to form result                          MOVOF      //    overwrite the OF position of Bop to                                           form result                                                  MOVDF      //    overwrite the DF position of Bop to                                           form result                                                  BSF        3'b011  //    bit scan forward                                     BSR        3'b100  //    bit scan reverse                                     BTS        3'b101  //    bit test and set                                     BTR        3'b110  //    bit test and reset                                   BTC        3'b111  //    bit test and complement                              ______________________________________                                    

Multiply Group

    ______________________________________                                        MUL16     3'b000  //    R(word) <- A * B(byte)                                IMUL16    3'b001  //    R(word) <- A * B(byte)                                INUL16S   3'b010  //    R(word) <- A * B(word)                                MUL32     3'b011  //    R(low word) <- A * B(word);                                                   allocate high word with MULH                          IMUL32    3'b011  //    R(low word) <- A * B(word);                                                   allocate high word with MULH                          IMUL32S   3'b011  //    R(dword) <- A * B(dword)                              MUL64     3'b011  //    R(low dword) <- A * B(dword);                                                 allocate high dword with MULH                         IMUL64    3'b011  //    R(low dword) <- A * B(dword);                                                 allocate high dword with MULH                                           //    if result fits in A then CF=OF=0                                              else CF=OF=1; SF=ZF=AF=PF=xxx                         ______________________________________                                    

DIV Group

    ______________________________________                                        DIV       3'b000   //    p=1001, g=110 <-when SDF=0                                              //    p=0110, g=101 <-when SDF= 1                          DIVL      3'b001   //    p=1001, g=110 <-when SDF= 0                                             //    p=0110, g=101 <-when SDF= 1                          DIV0      3'b010                                                              DIVREM    3'b011   //    p=1001, g=110 <-when SDF= 0                                             //    p=1100, g=111 <-when SDF= 1                          DIVCMP    3'b100   //    p=0110, g=101                                        DIVQ      3'b101   //                                                         DIV.sub.-- X0                                                                           3'b110   //    not used                                             DIV.sub.-- X1                                                                           3'b111   //    not used                                             ______________________________________                                    

IDIV Group

    ______________________________________                                        IDIVSGN      3'b000    //      p=0110, g=101                                  IDIVCMP      3'b001    //      p=0011, g=111                                                         //      p=0011, g=111                                  IDIVDEND0    3'b010    //      p=0011, g=111                                  IDIVDEND1    3'b011    //      p=0011, g=111                                  IDIVSOR      3'b100    //      p=0011, g=111                                  IDIVQ        3'b101    //      p=0011, g=111                                  IDIV.sub.-- X0                                                                             3'b110    //      not used                                       IDIV.sub.-- X1                                                                             3'b111    //      not used                                       ______________________________________                                    

The following two groups of encodings indicate what the RES bus of thefunctional unit contains.

STATUS Signals for RES bus

    ______________________________________                                         10:0! INSLSB indicates segment information for LSSEC                         F0BRN.sub.-- ADR;                                                                           assert proper branch status                                     F0BRN.sub.-- NT;                                                                            assert proper branch status                                     F0BRN.sub.-- OK;                                                                            assert proper branch status                                     F0BRN.sub.-- T.sub.-- ADR;                                                                  assert proper branch status                                     F0BRN.sub.-- T.sub.-- OK;                                                                   assert proper branch status                                     F0STAT.sub.-- LD;                                                                           RES bus status - load linear address                            F0STAT.sub.-- MUL;                                                                          RES bus status - mulitplier                                     F0STAT.sub.-- NONE;                                                                         RES bus status - no result                                      F0STAT.sub.-- OPST;                                                                         RES bus status - operand data for a                                           store                                                           F0STAT.sub.-- SRB;                                                                          RES bus status - SRB information                                F0STAT.sub.-- ST;                                                                           RES bus status - store linear address                           F0STAT.sub.-- VAL;                                                                          RES bus status - valid                                           10:0! INSLSB;                                                                              LSSEC segment MUX                                                2:0! DTAG;   destination tag                                                 ______________________________________                                    

Operand Steering Section Signal List

Inputs from the REGF and FIROB

    ______________________________________                                        FLAGCFDATn;    CF flag from FIROB                                             FLAGXFDATn;    CF flag from FIROB                                             RESET;         resets all processes in the multiplier                                        and GRNT.sub.-- CNT                                            ROBDCKnDAT1;   use the data from FIROB for pointer 1                          ROBDCKnDAT2;   use the data from FIROB for pointer 2                          SWAPPTR1W2;    swap pointer 1 data with pointer 2 data                        VATIn;         A TAG is valid - don't use data for                                           Aoperand                                                       VBTIn;         B TAG is valid - don't use data for                                           Boperand                                                       VCFTIn;        CF TAG is valid - don't use data for                                          carry flag                                                     VFLAGCFnD;     valid flag data on the CF flag bus                             VFLAGXFnD;     valid flag data on the status flag bus                         VSFTIn;        XF TAG is valid - don't use data for                                          status flags                                                    2:0! DESTAGn; destination tag which indicates the ID                                        of entry                                                        31:0! DISPIMM;                                                                              displacement and immediate data                                               (immediate will go to Bop bus)                                  31:0! RBnDAT1;                                                                              FIROB data for pointer1                                         31:0! RDnREG1;                                                                              data from the register read of pointer                                        1                                                               31:0! RDnREG2;                                                                              data from the register read of pointer                                        2                                                               3:0! CONSTD;  four bits of constant offsets for                                             PUSHes, POPs, and FPU linear addresses                          4:0! ATAGIn;  A tag                                                           4:0! BTAGIn;  B tag                                                           4:0! CFTAGIn; CF tag                                                          4:0! SFTAGIn; XF tag                                                         ______________________________________                                    

Inputs from the decode/ucode

    ______________________________________                                        RD.sub.-- DEP.sub.-- DS;                                                                     entry specific read bit for DS; entry                                         wait if both read and write are set                            RD.sub.-- DEP.sub.-- ES;                                                                     entry specific read bit for ES; entry                                         wait if both read and write are set                            RD.sub.-- DEP.sub.-- FS;                                                                     entry specific read bit for FS; entry                                         wait if both read and write are set                            RD.sub.-- DEP.sub.-- GS;                                                                     entry specific read bit for GS; entry                                         wait if both read and write are set                            VALPOS;        valid entry going from op steer to RS2                         WRDS;          global bit for write to DS                                     WRES;          global bit for write to ES                                     WRFS;          global bit for write to FS                                     WRGS;          global bit for write to GS                                      3:0! FLGEQ.sub.-- CC.sub.-- CHK;                                                            flag equation or condition code checks                          3:0! OPSIZE;  size position of result (dword, word,                                         ah, al)                                                         3:0! SEGSEL;  LSSEC segment MUX select                                        4:0! PROTOCOL;                                                                              determines protocol (ie: LOS, MUL,                                            DIV, etc.) RQLAC AXBLAC                                         6:0! OPCODE2; this is for extra control of vector 2                                         by the fast path (not for ucode)-                                             might this remove later                                         6:0! OPCODE;  the main opcode for both fast path                                            and ucode                                                      ______________________________________                                    

Outputs for the Reservation Station Control

    ______________________________________                                        MULCYC2X3N;     number of multiply cycles is 2, else                                          use 3                                                         RSRD.sub.-- DEP.sub.-- DS;                                                                    entry specific read bit for DS; entry                                         wait if both read and write are set                           RSRD.sub.-- DEP.sub.-- ES;                                                                    entry specific read bit for ES; entry                                         wait if both read and write are set                           RSRD.sub.-- DEP.sub.-- FS;                                                                    entry specific read bit for FS; entry                                         wait if both read and write are set                           RSRD.sub.-- DEP.sub.-- GS;                                                                    entry specific read bit for GS; entry                                         wait if both read and write are set                            10:0! RSPROTOCOL;                                                                            determines protocol (ie: LOS, SRB,                                            DIV, etc.)                                                    ______________________________________                                    

Outputs for the Reservation Station Entries

    ______________________________________                                        FLGCFDATn;     CF flag from FIROB                                             FLGXFDATn;     CF flag from FIROB                                             VATn;          A TAG is valid - don't use data for                                           Aoperand                                                       VBTn;          B TAG is valid - don't use data for                                           Boperand                                                       VCFTn;         CF TAG is valid - don't use data for                                          carry flag                                                     VFLGCFnD;      valid flag data on the CF flag bus                             VFLGXFnD;      valid flag data on the status flag bus                         VSFTn;         XF TAG is valid - don't use data for                                          status flags                                                    10:0! INSSEG; LSSEC segment MUX select                                        2:0! CDTAG;   current destination tag for the                                               incoming opcode                                                 31:0! AOPND;  A operand                                                       31:0! BOPND;  B operand                                                       31:0! DSPR;   displacement                                                    3:0! CONST;   constant offset value for PUSHes,                                             POPes, FPU linear addresses                                     4:0! ATAGn;   A tag                                                           4:0! BTAGn;   B tag                                                           4:0! CFTAGn;  CF tag                                                          4:0! SFTAGn;  XF tag                                                         ______________________________________                                    

Outputs for control store--shared for both vectors (for FNCU0 only)

    ______________________________________                                        F0CTLST.sub.-- VWR;                                                                          valid control store write next cycle                           F0DIV0;        initial divide opcode                                          F0DIV;         divide opcode                                                  F0DIVCMP;      divide opcode                                                  F0DIVL;        divide opcode                                                  F0DIVQ;        divide opcode                                                  F0DIVREM;      divide opcode                                                  F0DIVSGN;      divide opcode                                                  F0IDIVCMP;     divide opcode                                                  F0IDIVDEND0;   divide opcode                                                  F0IDIVDEND1;   divide opcode                                                  F0IDIVSOR;     divide opcode                                                  F0SELOPA;      The initial quotient is from operand A                                        else from adder output                                         F0UPDDF;       update division direction bit                                  F0UPDQ;        update division quotient                                       SELSQXOPA;     indicates that quotient gets shifted                                          quotient slave instead of adder or OPA                          3:0! F0CTLST.sub.-- WR;                                                                     indicates which control store vector                                          to use                                                          7:0! F0CTLST.sub.-- USE;                                                                    indicates which control store vector                                          to use                                                         ______________________________________                                    

Outputs for control store--shared for both vectors FNCU1, FNCU2, andFNCU3

    ______________________________________                                        F0CTLST.sub.-- VWR;                                                                           valid control store write next cycle                           3:0! F0CTLST.sub.-- WR;                                                                      indicates which control store vector                                          to use                                                         7:0! F0CTLST.sub.-- USE;                                                                     indicates which control store vector                                          to use                                                         7:0! FRESMUX;  select lines for MUX to RES bus                               ______________________________________                                    

Outputs for control store--vector 0 ; no shifts allowed

    ______________________________________                                        AXBLAC0;     indicates if operand A or B is used                                           for linear addr calc                                             F0ADD.sub.-- F0;                                                                           addition type flag generation                                    F0BCD.sub.-- FC0;                                                                          BCD clear type flag generation                                   F0BCD.sub.-- FS0;                                                                          BCD set type flag generation                                     F0CIN0;      carry-in from the control block                                  F0CLD.sub.-- F0;                                                                           clear direction flag generation                                  F0CMC.sub.-- F0;                                                                           complement carry type flag generation                            F0COMPB0;    complement the B operand                                         F0COND.sub.-- CODE0;                                                                       evaluate condition codes                                         F0DAA.sub.-- FS0;                                                                          DAA type flag generation                                         F0FLAGCF0;   opcode requires using a version of                                            EFLAGCF                                                          F0LOG.sub.-- F0;                                                                           logical type flag generation                                     F0MOVCC0;    MOVCC instruction                                                F0MUXCIN0;   carry-in MUX                                                     F0MUXDF0;    direction flag MUX                                               F0SETCC0;    SETCC instruction                                                F0SETCF0;    ADC used true version of EFLAGSCF else                                        ˜EFLAGSCF                                                  F0STD.sub.-- F0;                                                                           store direction flag generation                                  F0UNC.sub.-- BRN0;                                                                         unconditional branch evaluation                                  FISTAT.sub.-- BRN0;                                                                        assert proper branch status                                      FISTAT.sub.-- LD0;                                                                         RES bus status - load linear address                             FISTAT.sub.-- MUL0;                                                                        RES bus status - multiplier                                      FISTAT.sub.-- NONE0;                                                                       RES bus status - no result                                       FISTAT.sub.-- OPST0;                                                                       RES bus status - operand data for a                                           store                                                            FISTAT.sub.-- SRB0;                                                                        RES bus status - SRB information                                 FISTAT.sub.-- ST0;                                                                         RES bus status - store linear address                            FISTAT.sub.-- VAL0;                                                                        RES bus status - valid                                           RQLAC0;      linear address calculation                                       SIGNXA0;     sign extend the A operand                                        SIGNXB0;     sign extend the B operand                                        SIGNXD0;     sign extend the D operand                                        USEOFFSET0;  use offset for PUSH, POP, or FPU lin                                          adr calc                                                         ZEROXA0;     zero extend the A operand                                        ZEROXB0;     zero extend the B operand                                        ZEROXD0;     zero extend the D operand                                         1:0! AHBYTE0;                                                                             alignment select bits0;move bits 15:8                                         to 7:0                                                            2:0! F0GIN0;                                                                              generate style inputs (used here to                                           indicate type of operation)                                       3:0! F0PIN0;                                                                              propagate style inputs (used here to                                          indicate type of operation)                                       3:0! OPSIZIN0;                                                                            size position of result (dword, word,                                         ah, al)                                                           4:0! F0SCALE0;                                                                            scale for SIB: 8,4,2,1,0                                          7:0! FRESMUX0;                                                                            select lines for MUX to RES bus                                  ______________________________________                                    

Outputs for control store--vector 1; shifts allowed

    ______________________________________                                        AXBLAC1;     indicates if operand A or B is used                              for linear addr calc                                                          F0ADD.sub.-- F1;                                                                           addition type flag generation                                    F0BCD.sub.-- FC1;                                                                          BCD clear type flag generation                                   F0BCD.sub.-- FS1;                                                                          BCD set type flag generation                                     F0CIN1;      carry-in from the control block                                  F0CLD.sub.-- F1;                                                                           clear direction flag generation                                  F0CMC.sub.-- F1;                                                                           complement caryy type flag generation                            F0COMPB1;    complement the B operand                                         F0COND.sub.-- CODE1;                                                                       evaluate condition codes                                         F0DAA.sub.-- FS1;                                                                          DAA type flag generation                                         F0FLAGCF1;   opcode requires using a version of                               EFLAGSCF                                                                      F0LOG.sub.-- F1;                                                                           logical type flag generation                                     F0MOVCC1:    MOVCC instruction                                                F0MUXCIN1;   carry-in MUX                                                     F0MUXDF1;    direction flag MUX                                               F0SETCC1;    SETCC instruction                                                F0SETCF1;    ADC uses true version of EFLAGSCF else                           ˜EFLAGSCF                                                               F0STD.sub.-- F1;                                                                           store direction flag generation                                  F0UNC.sub.-- BRN1;                                                                         unconditional branch evaluation                                  FISTAT.sub.-- BRN1;                                                                        assert proper branch status                                      FISTAT.sub.-- LD1;                                                                         RES bus status - load linear address                             FISTAT.sub.-- MUL1;                                                                        RES bus status - mulitplier                                      FISTAT.sub.-- NONE1;                                                                       RES bus status - no result                                       FISTAT.sub.-- OPST1;                                                                       RES bus status - operand data for a                              store                                                                         FISTAT.sub.-- SRB1;                                                                        RES bus status - SRB information                                 FISTAT.sub.-- ST1;                                                                         RES bus status - store linear address                            FISTAT.sub.-- VAL1;                                                                        RES bus status - valid                                           RQLAC1;      linear address calculation                                       SHFBCMP1;    indicates a Bit Test and Complement                              SHFBITEST1;  a select signal for any Bit Test                                 opcode                                                                        SHFBITOPA1;  a select signal for only the BTEST                               (BT) opcode                                                                   SHFBSF1;     select signal for Bit Scan Forward                               SHFBSR1;     select signal for Bit Scan Reverse                               SHFBSXR1;    indicates a set/reset for bit test                               instruction                                                                   shfeflag1;   merge Bop & all flags into result                                SHFFLGDF1;   overwrite the DF position of Bop to                              form result                                                                   SHFFLGOF1;   overwrite the OF position of Bop to                              form result                                                                   SHFLAHF1;    load Flags into AH opcode                                        SHFLEFT1;    indicates RCL, ROL, or SHL opcode                                SHFRIGHT1;   indicates RCR, ROR, SAR, or SHR opcode                           SHFROT1;     indicates a rotate or SHL opcode                                 SHFSAHF1;    store AH into Flags opcode                                       SHFSELSF31.sub.-- 1;                                                                       set the sign flag to the shifter                                 output bit 31                                                                 SHFSETCF1;   set carry flag for RCL & RCR equal to                            EFLAGSCF                                                                      SHF.sub.-- UPD.sub.-- RC1;                                                                 select for updating the local carry                              flag                                                                          SIGNXA1;     sign extend the A operand                                        SIGNXB1;     sign extend the B operand                                        SIGNXD1;     sign extend the D operand                                        USEOFFSET1;  use offset for PUSH, POP, or FPU lin                             adr calc                                                                      ZEROXA1;     zero extend the A operand                                        ZEROXB1;     zero extend the B operand                                        ZEROXD1;     zero extend the D operand                                         1:0! AHBYTE1;                                                                             alignment select bits0;move bits 15:8                            to 7:0                                                                         21:0! SHFSELPRE1;                                                                         the preshift mux select signals                                   2:0! SHFSELZF1;                                                                           determines which groups of bit to use                            for setting the zero flag                                                      3:0! F0PIN1;                                                                              propagate style inputs (used here to                             indicate type of operation)                                                    3:0! OPSIZEIN1;                                                                           size/position of result (dword, wor,                             ah al)                                                                         3:0! SHFSELOF1;                                                                           determines which groups of bit to use                            for setting the overflow flag                                                  3:0! shfselout1;                                                                          indicates final byte alignment for the                           shifter output                                                                 7:0! FREMUX1;                                                                             select lines for MUX to RES bus                                   &:0! SHFSELCF1;                                                                           determines which shifter bits to use                             to set the carry flag                                                          9:0! SHFSELCNT1;                                                                          mux select signals determining source                            of shifter count value                                                        ______________________________________                                    

Functional Unit (FNCU)

This section covers the functional unit (FNCU) which contains the ALU,branch evaluation, and shifter. The six sub blocks of the FNCU are theCBVS (control bit vector store), the OPSEL (operand selection andalignment), the ALU (for logical and arithmetic operations), the SHFT(shifter), the FLGCC (flag generations, branch/condition codeevaluation), and the MXDVR (output multiplier, alignment, and drivers).An overview of basic flow (shown in FIG. 31) is that all opcode, tag,and operand information goes to the operand steering section where theappropriate information is determined and driven onto the A operand, Boperand, A tag, B tag, and other busses. Also some decode is done in theoperand steering section for such information as the P and G bits forthe given opcode. The operand steering section drives RS2 of thereservation station block. Reservation station control logic latches intags at the beginning of the cycle, compares to determine if forwardingcan be done, and prioritizes which reservation station will drive theFNCU. At the beginning of the next cycle the DTAG for the opcode to beexecuted that cycle is driven out to allow other reservation stations toforward during the following cycle. Then a short delay is given to allowthe latched in MUX controls, the P and G bits, and other inputinformation to set up the adder or shifter.

Then if needed, the B operand can be aligned (bits 15:8 to 7:0, orscaled 1×,2×,4×,8×) or complemented. The execution (add, and, shift,etc) then takes place, and the output of the adder is driven ASAP forbits 14:0 to the RESLA bus.

Two control bit vectors can be used by each entry in the reservationstation. These vectors come from decoding logic in the operand steeringblock and the Idecode block. For example one vector could be for linearaddress calculation while the second is for the actual operation.Another example is that one vector could represent an ADD while theother represents a SUB and the division direction flag would be used tochoose the vector to use. The FDRES bus drives (with minimum capacitiveload) to the input MUXes of the FNCUs. The FDRES bus also is buffered tobecome the RES (result) bus which goes to the reservation stations (notfor immediate forwarding), LSSEC, and the FIROB data block.

Branch evaluations are performed in the FNCU with a branch address beinggenerated along with a branch status. Logical load/store address arecalculated in the FNCU and sent out on the LSSEC bus (also RESLA) buswith a corresponding encoding on the status bus to identify them.Multiply operands are sent from the reservation station onto the Aoperand and B operand busses and then onto the shared tristate SAOPNDand SBOPND busses (shared A operand) to the multiplier and the floatingpoint unit.

RFLAGs (result flags: {ZF,AF,PF,SF,DF,OF}) are generated the during thecycle following the execution of the corresponding opcode. The carryflag {CF} is generated the same cycle that the result gets generated. Ablock diagram of a functional unit is shown as FIG. 32.

Basic Flow of Operands to the FNCU

This section will cover the general flow of operands meeting up withtheir opcodes. For more detailed descriptions, refer to the sections onoperand steering and on dependency checking. Forwarding is used here toindicate that a unit other than the FIROB has watched the DTAG bussesand LSTAG busses for a renamed value (eg: EAX renamed to DTAG=2₋₋ 0) andlatched the value off of the corresponding result bus. The operandsteering blocks, the 12 reservation stations, and the FNCU can allreceive data by forwarding; the multiplier cannot. In some cases such asreceiving load data from the LSRES busses, both the FNCU and thereservation station can receive the forwarded data. The FNCU willexecute the opcode using the forwarded data, and the reservation stationthat issued that opcode would hold the entry for an extra cycle just incase a miss or way misprediction signal comes back. In the followingexample, the B, C, and D instructions are all waiting for data that isgenerated by the A instruction.

Example

    ______________________________________                                        instruction                                                                   ID             instruction                                                    ______________________________________                                        A              DTAG=1.sub.-- 0 ADD EAX,immed                                  B              DTAG=2.sub.-- 1 SUB EBX,EAX                                    C              DTAG=3.sub.-- 0 XOR ECX,EAX                                    D              DTAG=4.sub.-- 3 OR EDX,EAX                                     ______________________________________                                        Cycle                                                                         Stage   1        2        3      4      5                                     ______________________________________                                        dep. chec                                                                             inst A   inst B   inst C inst D                                       decode  dtag= 1.sub.-- 0                                                                       dtag= 2.sub.-- 1                                                                       dtag= 3.sub.-- 0                                                                     dtag= 4.sub.-- 3                             REGF access      btag= 1.sub.-- 0                                                                              EAX from                                                                      regf                                         operand          inst A   inst B inst C inst D                                steering         dtag= 1.sub.-- 0                                                                       dtag= 2.sub.-- 1                                                                     dtag= 3.sub.-- 0                                                                     dtag= 4.sub.--3                                                 btag= 1.sub.-- 0                                                                     EAX from                                                                             EAX from                                                               FIROB  regf                                  execute                   inst A inst B inst C                                                          dtag= 1.sub.-- 0                                                                     dtag= 2.sub.-- 1                                                                     dtag= 3.sub.-- 0                                                for-                                                                          warding                                             writeback                        inst A inst B                                (same as dep                     dtag= 1.sub.-- 0                                                                     dtag= 2.sub.-- 1                      check stage)                                                                  ______________________________________                                    

The B instruction uses forwarding to get the operand steering section toreceive the EAX result from the A instruction. The C instruction getsits EAX data from the FIROB, and the D instruction gets its EAX datafrom the REGF. The "A" result tags are driven through cycle 3 so thatany other reservation stations or the operand steering section candetect BTAG=1₋₋ 0 and latch the "A" result value at the end of cycle 3.No "A" result tag information is being broadcast in cycle 4, so the Cinstruction cannot receive EAX by forwarding. This example assumes thatthe FIROB is writing back every cycle; otherwise, the D instructionwould also get its EAX data from the FIROB if EAX had not been writtenback yet. The operand steering must have tag compare logic inside it toget the EAX operand.

ALU/Shift/Rotate/Branch Opcodes

The following encodings determine which flag equations to generate.Separate signals are sent from the Idecode to the FIROB to indicatewhich combination of the three flag groups gets written back by theFIROB to the EFLAGS0 (standard) or EFLAGS1 (scratch) registers.

Flag Equation Groups

    ______________________________________                                        ADD.sub.-- F                                                                             3'b000 //                                                                              flag equations for addition, sub,                                             etc.                                                      LOG.sub.-- F                                                                             3'b001 //                                                                              flag equations for logical                                CMC.sub.-- F                                                                             3'b010 //                                                                              flag equations for complement CF                          BCD.sub.-- FS                                                                            3'b011 //                                                                              flag equations for AAA, AAS                               BCD.sub.-- FC                                                                            3'b100 //                                                                              flag equations for AAA, AAS                               DAA.sub.-- FS1                                                                           3'b101 //                                                                              flag equations for DAA, DAS                               STD.sub.-- F                                                                             //       flag equations for STD                                    CLD.sub.-- F                                                                             //       flag equations for CLD                                    ______________________________________                                    

Basic ALU Group

    ______________________________________                                        ADD      3'b000 //                                                                              R <- A + B    p=1001, g=110                                 OR       3'b001 //                                                                              R <- A | B                                                                         p=1000, g=111                                 ADC      3'b010 //                                                                              R <- A + B + CF                                                                             p=1001, g=110                                 SBB      3'b011 //                                                                              R <- A - B + CF                                                                             p=0110, g=101                                 AND      3'b100 //                                                                              R <- A & B    p=1110, g=111                                 SUB      3'b101 //                                                                              R <- A - B    p=0110, g=101                                 XOR      3'b110 //                                                                              R <- A   B    p=1001, g=111                                 MULH     3'b111 //                                                                              this opcode is a NOP which holds                                              a res sta slot and waits for                                         //       the 2nd multiply result to                                                    request to drive data to the FNCU                           ______________________________________                                    

Branch and Extended ALU Group

(for BRN>>>FNCU receives: A<-EIP, disp<-rel, B<-pred addr)

    ______________________________________                                        JMPRI  3'b000 //                                                                              R <- A     register                                                                      indirect (uncond);                                                            source = r16,r32,  r16+d16!, or                                                r32+d32! & set R.sub.-- BRN.sub.-- ADR            JMP    3'b001 //                                                                              R <- A + disp                                                                            jump relative                                                                 (uncond) & set R.sub.-- BRN.sub.-- ADR             JMPCC  3'b010 //                                                                              R <- A + disp                                                                            jump relative if                                                              condition true & set                                                          R.sub.-- BRN.sub.-- OK,                                                       R.sub.-- BRN.sub.-- NT,                                                       or                                                                            R.sub.-- BRN.sub.-- ADR                            SETCC  3'b011 //                                                                              R(0) <- 1'b1                                                                             set result bit 0                                                              if condition true                                  ANDN   3'b100 //                                                                              R <- A & XB                                                                              p=1101, g=111                                      SUBR   3'b101 //                                                                              R <- B - A p=0110, g=011                                      DFADD  3'b110 //                                                                              R <- A + B p=1001, g=110 <-when                                                          EFLAGDF=0                                                 //       R <- A - B p=0110, g=101 <-when                                                          EFLAGDF=1                                          MOVCC  3'b111 //                                                                              R <- B     p=1010, g=111 <-when                                                          condition is true                                         //       R <- A     p=1010, g=111                                                                 ignore adder result and                                                       pass Aop when cond. false                          ______________________________________                                         // * note: JMP w/ R <- immed will be handled in the IFETCH for absolute       jump using immediate value                                               

Transfer Group

    ______________________________________                                        MOVSX     3'b000 // R <- B     p=1010, g=111 sign                                                            extend B                                       MOVZX     3'b001 // R <- B     p=1010, g=111 zero                                                            extend B                                       SIGNXA    3'b010 // R <- A     p=1100, g=111 sign                                                            extend A                                       ZEROXA    3'b011 // R <- A     p=1100, g=111 zero                                                            extend A                                       MOVDSPSX  3'b100 // R <- disp  p=1010, g=111 sign                                                            extend disp                                    MOVDSPZX  3'b101 // R <- disp  p=1010, g=111 zero                                                            extend disp                                    LEAA      3'b110 // R <- A+disp                                                                              p=1001, g=110                                                                 loads effective address                                                       (Aside)                                        LEAB      3'b111 // R <- B+disp                                                                              p=1001, g=110                                                                 loads effective address                                                       (Bside)                                        ______________________________________                                    

Shift Group

    ______________________________________                                        ROL        3'b000 //                                                                              rotate left                                               ROR        3'b001 //                                                                              rotate right                                              RCL.sub.-- 1                                                                             3'b010 //                                                                              rotate left through carry (1 bit                                              only)                                                     RCR.sub.-- 1                                                                             3'b011 //                                                                              rotate right through carry (1 bit                                             only)                                                     SHL        3'b100 //                                                                              shift left arithmetic or logical                          SHR        3'b101 //                                                                              shift right logical                                       RC0        3'b110 //                                                                              initial step of rotate through                                                carry sequence                                            SAR        3'b111 //                                                                              shift right arithmetic                                    ______________________________________                                    

Bit Group

    ______________________________________                                        BT        3'b000 //                                                                              bit test                                                   LAHF      3'b001 //                                                                              load flags into AH                                                            R <- flags                                                                    (sf:zf:00:af:00:pf:00:df)                                  SAHF      3'b010 //                                                                              store AH into flags R <- A;                                                   flags <- A(low byte)                                                          (sf:zf:00:af:00:pf:00:df)                                  MOVF      //       combine Bop and flag to form result                        MOVOF     //       overwrite the OF position of Bop to                                           form result                                                MOVDF     //       overwrite the DF position of Bop to                                           form result                                                BSF       3'b011 //                                                                              bit scan forward                                           BSR       3'b100 //                                                                              bit scan reverse                                           BTS       3'b101 //                                                                              bit test and set                                           BTR       3'b110 //                                                                              bit test and reset                                         BTC       3'b111 //                                                                              bit test and complement                                    ______________________________________                                    

Multiply Group

    ______________________________________                                        MUL16     3'b000 //                                                                              R(word) <- A * B(byte)                                     IMUL16    3'b001 //                                                                              R(word) <- A * B(byte)                                     IMUL16S   3'b010 //                                                                              R(word) <- A * B(word)                                     MUL32     3'b011 //                                                                              R(low word) <- A * B(word);                                                   allocate high word with MULH                               IMUL32    3'b011 //                                                                              R(low word) <- A * B(word);                                                   allocate high word with MULH                               IMUL32S   3'b011 //                                                                              R(dword) <- A * B(dword)                                   MUL64     3'b011 //                                                                              R(low dword) <- A * B(dword);                                                 allocate high dword with MULH                              IMUL64    3'b011 //                                                                              R(low dword) <- A * B(dword);                                                 allocate high dword with MULH                                        //       if result fits in A then CF=OF=0                                              else CF=OF=1; SF=ZF=AF=PF=xxx                              ______________________________________                                    

DIV Group

    ______________________________________                                        DIV         3'b000 //                                                                              p=1001, g=110 <-when SDF= 0                                          //       p=0110, g=101 <-when SDF= 1                              DIVL        3'b001 //                                                                              p=1001, g=110 <-when SDF= 0                                          //       p=0110, g=101 <-when SDF= 1                              DIV0        3'b010 //                                                         DIVREM      3'b011 //                                                                              p=1001, g=110 <-when SDF= 0                                          //       p=1100, g=111 <-when SDF= 1                              DIVCMP      3'b100 //                                                                              p=0110, g=101                                            DIVQ        3'b101 //                                                         DIV.sub.-- X0                                                                             3'b110 //                                                                              not used                                                 DIV.sub.-- X1                                                                             3'b111 //                                                                              not used                                                 ______________________________________                                    

IDIV Group

    ______________________________________                                        IDIVSGN          3'b000 //                                                                              p=0110, g=101                                                        //                                                           IDIVCMP          3'b001 //                                                                              p=0011, g=111                                                        //       p=0011, g=111                                       IDIVDEND0        3'b010 //                                                                              p=0011, g=111                                       IDIVDEND1        3'b011 //                                                                              p=0011, g=111                                       IDIVSOR          3'b100 //                                                                              p=0011, g=111                                       IDIVQ            3'b101 //                                                                              p=0011, g=111                                       IDIV.sub.-- X0   3'b110 //                                                                              not used                                            IDIV.sub.-- X1   3'b111 //                                                                              not used                                            ______________________________________                                    

The following two groups of encodings indicate what the RES bus of thefunctional unit contains.

STATUS Signals for RES Bus

    ______________________________________                                         10:0! INSLSB                                                                              indicates segment information for LSSEC                          F0BRN.sub.-- ADR;                                                                          assert proper branch status                                      F0BRN.sub.-- NT;                                                                           assert proper branch status                                      F0BRN.sub.-- OK;                                                                           assert proper branch status                                      F0BRN.sub.-- T.sub.-- ADR;                                                                 assert proper branch status                                      F0BRN.sub.-- T.sub.-- OK;                                                                  assert proper branch status                                      F0STAT.sub.-- LD;                                                                          RES bus status - load linear                                                  address                                                          F0STAT.sub.-- MUL;                                                                         RES bus status - multiplier                                      F0STAT.sub.-- NONE;                                                                        RES bus status - no result                                       F0STAT.sub.-- OPST;                                                                        RES bus status - operand data for a                                           store                                                            F0STAT.sub.-- SRB;                                                                         RES bus status - SRB information                                 F0STAT.sub.-- ST;                                                                          RES bus status - store linear                                                 address                                                          F0STAT.sub.-- VAL;                                                                         RES bus status - valid                                            10:0! INSLSB;                                                                             LSSEC segment MUX                                                 2:0! DTAG;  destination tag                                                  ______________________________________                                    

ALU Sub-Block

This sub-block is used for calculating linear addresses needed by theLSSEC, performing comparisons, and for computing arithmetic operations.Operations such as DAA, DAS, AAA, AAS, AAM, and AAD will be done inmicrocode and will not receive direct support from the ALU except maybefor flag handling. The adder uses propagates and generates which aresetup in the reservation stations based on the type of operation.

Shifter Sub Block

The FNCU contains a barrel shifter which shifts the A operand by theamount indicated on the B operand. This unit handles bit manipulationinstructions, counting leading zeroes/ones, and shifts and rotates. RCLand RCR by one is handled but microcode is used for RCL and RCR by morethan one. There is a SHFT block which contains the data path portion ofthe shifter while the control logic is contained along with the ALUcontrol logic in the FNCUCTL block. The output of the shifter hastristate drivers which drive the result bus. At the beginning of thecycle the opcode type is latched in from the reservation station andthis information is used to decide if the ALU or the SHFT block willdrive the result bus.

Branch Evaluation

For branch instructions, the EIP value is sent on the A operand buswhile the relative jump value is sent on the displacement bus. The Boperand bus is used for the predicted address. For "jump near indirect"the jump address is the r/m16 or r/m32 loaded into the EIP. The controlsection of the functional unit evaluates whether the condition codes andthe forwarded flags result in a branch taken or not taken. The samelogic is also used for the SETcc opcode. Branch prediction informationis sent by the branch prediction unit to the dispatch unit and then tothe IFCORE. Four different cases for conditional branches are handled.

    ______________________________________                                        Cond Code                                                                             Predicted                                                                              ALU              Status                                      ______________________________________                                        not taken                                                                             not taken                                                                              EIP (Aopnd) + displacement                                                                     R.sub.-- BRN.sub.-- OK                      not taken                                                                             taken    EIP (Aopnd) + displacement                                                                     R.sub.-- BRN.sub.-- NT                      taken   not taken                                                                              EIP (Aopnd) + displacement                                                                     R.sub.-- BRN.sub.-- ADR                     taken   taken    EIP (Aopnd) + displacement                                                                     R.sub.-- BRN.sub.-- ADR                     ______________________________________                                    

For the case of a predicted taken branch, either the R₋₋ BRN₋₋ NT statusshows that the branch was not taken, or the R₋₋ BRN₋₋ ADR indicates thatthe actual branch address will be compared to the predicted addressusing the flag evaluation unit and the result of that compare will beseen next cycle.

1st cycle RES₋₋ STA assign branch evaluation to FNCU for next cycle

2nd cycle EIP+d32; evaluate condition codes and flags (last cycle forpredicted not taken)

3rd cycle Flags block compares the predicted branch address with theactual branch address. ZF=1 indicates that the two addresses are thesame.

Output Drivers (Buffers/MUX) Sub Block

This block muxes the output data from either the adder, shifter,exception vector, or the MULTFP bus (for FNCU). The linear addresscalculation uses the adder. Some bits, RESLAn(14:5), for the linearaddress are sent out to the data cache early while the entire resultgoes out on FDRESn(31:0) at the end of the cycle. The purpose of theFDRESn bus is to forward quickly to the input MUXes of all fourfunctional units. RESn is a buffered version of FDRESn which is lesstime critical than FDRESn. The RESn bus drives the LSSEC, the FIROB, andall of the reservation stations. To determine which sub-block will beMUXed onto the FDRES bus, the opcode group indicator, the multiplyrequest (also MULH mnemonic), exception information, linear addressindication, and branch information are used.

Condition Flags Sub Block

Three flag groups are defined as follows: {CF}, {OF,ZF,SF,AF,PF,DF}, and{TF,IF,IOPL,NT,RF,VM,AC,VIF,VIP,ID}. These flags are generated duringthe next cycle after the FNCU executes. The carry flag will simply bethe carry out from cell 31, 15, or 7 based on operand size. For the caseof a subtract opcode, the carry flag is used to represent the borrow andis the inverted bit from the bit 31, 15, or 7 carry out. The parity flagis the even parity on the lowest byte which requires about2*Order(log2n)=6 gate levels. The sign flag is a mux of the highestorder bit (31, 15, or 7). These flags are latched by the FIROB at thebeginning of the next cycle and also forwarded to any FNCU looking forthem. The dependency checking will be done individually on each of thethree groups. The first two flag groups allow forwarding of the resultswhile the last group will require serialization when a dependency isdetected. The reservation station receives an early use signal fromdecode which corresponds to the two flag groups. This means that for agiven opcode which is sitting in a reservation station entry and waitingon flags, an early use entry can issue the cycle right after the flagmatch is detected. Any other reservation station entry must wait anadditional cycle before issuing to the functional units. The additionalcycle is because the flags are generated the cycle after the functionalunit executes. For instructions like ADC, the carry flag will be neededat the beginning of the cycle. For instructions like jump, the flags andcondition codes start evaluating during the middle of the cycle. Theconditional jumps, MOVCC, and SETCC opcodes get their flags forwardedduring mid-cycle while all others wait until the beginning of oneadditional cycle.

In addition to the standard EFLAGS register (EFLAGS0) there is also ascratch EFLAGS register (EFLAGS1) for microcode. The FNCU knows nothingabout these two different EFLAGS registers. It simply gets forwardedflag data off the RFLAGSn busses or off its flag input bus. The cycleafter every opcode execution, the flags block generates a complete setof flags according to the flag equations mnemonics generated by theIDECODE.

The flag section also takes the B operand (predicted branch address) andcompares it the result (EIP+d32). If the two are equal for a branchevaluation, the ZF (zero flag) is set during the next cycle. If the ZFis not set, then the FIROB knows that for a predicted branch taken andactual branch taken case, the IFETCH did not branch to the correctaddress.

Flag Equations for Functional Units

(these are what the FNCU generates; not what the FIROB writesback)

    ______________________________________                                        ADD.sub.-- F flag equations for addition, sub, etc.                           if (Byte) : OF = CO(7)   CO(6) ; SF = R(7) ;                                  ZF = ˜| R(7:0) ; AF = CO(3); PF = ˜  R(7:0); CF =        CO(7); DF =                                                                   DF(in)                                                                        if (Word) : OF = CO(15)   CO(14); SF = R(15);                                 ZF = ˜| R(15:0); AF = CO(3); PF = ˜  R(7:0); CF =        CO(15);                                                                       DF = DF(in)                                                                   if (Dword): OF = CO(31)   CO(30); SF = R(31);                                 ZF = ˜| R(31:0); AF = CO(3); PF = ˜  R(7:0); CF =        CO(31)                                                                        DF = DF(in)                                                                   * CF is inverted when representing borrow                                     instead of carry out                                                          LOG.sub.-- F flag equations for logical                                       if (Byte) : OF = 0; SF = R(7) ;                                               ZF = ˜| R(7:0) ; AF = xxx; PF = ˜  R(7:0); CF = 0;       DF =                                                                          DF(in)                                                                        if (Word) : OF = 0; SF = R(15);                                               ZF = ˜| R(15:0); AF = xxx; PF = ˜  R(7:0); CF = 0;       DF =                                                                          DF(in)                                                                        if (Dword) : OF = 0; SF = R(31);                                              ZF = ˜| R(31:0); AF = xxx; PF = ˜  R(7:0); CF = 0;       DF =                                                                          DF(in)                                                                        CMC.sub.-- F flag equations                                                   if (Byte) : OF = CO(7)   CO(6) ; SF = R(7) ;                                  ZF = ˜| R(7:0) ; AF = CO(3); PF = ˜  R(7:0); CF =        ˜CF(in);                                                                DF = DF(in)                                                                   if (Word) : OF = CO(15)   CO(14); SF = R(15);                                 ZF = ˜| R(15:0); AF = CO(3); PF = ˜  R(7:0); CF =        ˜CF(in);                                                                DF = DF(in)                                                                   if (Dword): OF = CO(31)   CO(30); SF = R(31);                                 ZF = ˜| R(31:0); AF = CO(3); PF = ˜  R(7:0); CF =        ˜CF(in);                                                                DF = DF(in)                                                                   BCD.sub.-- FS flag equations                                                  if (Byte) : OF = CO(7)   CO(6) ; SF = R(7) ;                                  ZF = ˜| R(7:0) ; AF = 1; PF = ˜  R(7:0); CF = 1; DF      DF(in)                                                                        BCD.sub.-- FC flag equations                                                  if (Byte) : OF = CO(7)   CO(6) ; SF = R(7) ;                                  ZF = ˜| R(7:0) ; AF = 0; PF = ˜  R(7:0); CF = 0; DF      =                                                                             DF(in)                                                                        DAA.sub.-- FS1 flag equations                                                 if (Byte) : OF = CO(7)   CO(6) ; SF = R(7) ;                                  ZF = ˜| R(7:0) ; AF = 1; PF = ˜  R(7:0); CF =            CF(input) | CO(7)                                                    DF = DF(in)                                                                   * CF is inverted when representing borrow                                     instead of carry out                                                          STD.sub.-- F flag equations                                                   OF = OF(in); SF = SF(in); ZF = ZF(in); AFS =                                  AF(in); PF = PF(in); CF = CF(in); DF = 1;                                     CLD.sub.-- F flag equations                                                   OF = OF(in); SF = SF(in); ZF = ZF(in); AFS =                                  AF(in); PF = PF(in); CF = CF(in); DF = 1;                                     ______________________________________                                         *note: the flag equations mnemonic sent to the FNCU is ignored during LAH     and SAHF; the FNCU generates all flags per the flag mnemonic but the FIRO     decides which flags are actually written back                            

Method of Sending Data Cache Linear Addresses to LSSEC

The FNCU can be used to calculate a linear address using thedisplacement bus and either the A or B operand bus. The result is sentto the LSSEC using the RES bus. This assumes that a segment with nooffset is being used. When the segment offset needs to be added in, theLSSEC handles the segment offset addition. AXBnLAC lets the functionalunit know if the AOPND or BOPND bus will be added to the displacementbus. RQnLAC indicates that the functional unit is performing a linearaddress calculation. In the case of EAX! which has no displacement, thedecode block will have sent a zero constant on the displacement bus.

Linear address calculations will be handled as follows:

1) SIB will be mapped to two dispatch positions such that the first oneadds the base with the scaled index; this result goes to the seconddispatch position which operates like a regular opcode but gets resultsfrom the first (eg: OPCODE 1st₋₋ result+d32!,r32. The reason fordividing the SIB into two opcodes is to get dependency checkingperformed on three source operands.

2) All other linear address calculations (one forwarded variable and onedisplacement) go to the reservation station where they wait for theforwarded variable. Then for a Load or Load followed by store, thereservation station keeps its information with the forwarded variableand displacement go to the functional unit to be added. Then the resultbus is used to send the Load linear address to the LSSEC. The LSSEC mustbe able to latch one linear address from each functional unit during acycle. Dispatch/decode makes sure that no more than 8 loads and storesare pending at any one time. If a non-zero segment offset is present,four adders are used in the LSSEC to complete the calculation fromlogical to linear address.

3) For a Load and Store, once the Load linear address is calculated bythe functional unit this linear address is both saved for the subsequentstore and also sent to the LSSEC for the Load.

In the case of a load, the reservation station remains valid and waitsfor the LSSEC to return the data. The status bus indicates whether theRES bus contains an F0STAT₋₋ NONE, F0STAT₋₋ LD, etc. The LSSEC can doLoads in any order but also checks them with the Store buffer (inparallel) for any dependencies. DTAG Information is sent from thedecode/dispatch directly to the LSSEC to indicate the true store order,since the functional units (FNCU) may give stores to the LSSEC in out oforder sequence.

The typical flow for forwarding from a Load-OP instruction followed byan operation only instruction is shown. Note that after the BTAG changesfrom its original value (don't care for this case) to the DTAG value ofits opcode. When the LSSEC sends back the data the DTAG of the receivingopcode will be on the LSTAG bus.

    ______________________________________                                        example: ADD EAX,  ECX+d32!                                                                            send to issue position 0                                      XOR EBX,EAX     send to issue position 2                             RES.sub.-- STA                                                                         RES.sub.-- STA                                                       DTAG=1.sub.-- 0                                                                        DTAG=2.sub.-- 2                                                      BTAG=xxx BTAG=1.sub.-- 0                                                      calc LA  idle            idle                                                 FNCU0    FNCU2           LSSEC:SRB                                            .sub.-- RES0.sub.--                                                                    .sub.-- RES2.sub.--                                                                           .sub.-- LSRES.sub.--                                 B+disp                                                                        DTAG=1.sub.-- 0                                                               R.sub.-- LOAD                                                                 RES.sub.-- STA                                                                         RES.sub.-- STA                                                       DTAG=1.sub.-- 0                                                                        DTAG=2.sub.-- 2                                                      BTAG=1.sub.-- 0                                                                        BTAG=1.sub.-- 0 performs                                             idle     idle            load                                                 FNCU0    FNCU2           LSSEC:SRB                                            .sub.-- RES0.sub.--                                                                    .sub.-- RES2.sub.--                                                                           .sub.-- LSRES.sub.--                                                          LSTAG=1.sub.-- 0                                                              R.sub.-- LD.sub.-- DAT                               RES.sub.-- STA                                                                         RES.sub.-- STA                                                                DTAG=2.sub.-- 2                                                               BTAG=1.sub.-- 0                                                      execute  idle            idle                                                 FNCU0    FNCU2           LSSEC:SRB                                            .sub.-- RES0.sub.--                                                                    .sub.-- RES2.sub.--                                                                           .sub.-- LSRES.sub.--                                 execute                                                                       DTAG=1.sub.-- 0                                                               R.sub.-- VAL                                                                  RES.sub.-- STA                                                                         RES.sub.-- STA                                                       idle     execute         idle                                                 FNCU0    FNCU2           LSSEC:SRB                                            .sub.-- RES0.sub.--                                                                    .sub.-- RES2.sub.--                                                                           .sub.-- LSRES.sub.--                                          execute                                                                       DTAG=2.sub.-- 2                                                               R.sub.-- VAL                                                         ______________________________________                                    

Push/Pop Linear address calculations will be handled as follows:

For a PUSH, the first vector will subtract 2h or 4h from the ESP and theresult will be both latched by the FIROB as the new ESP and latched bythe LSSEC as the store linear address. The second vector is a move ofsome register (passes through the FNCU) to the LSSEC, and to memory. Apush memory will require two dispatch positions (see reservation stationdocument for more details).

For a POP, two dispatch positions are always required because there aretwo register destinations that must be updated. Vector 0 of the firstdispatch position passes the ESP value through the FNCU and the LSSEClatches it as a load linear address. The reservation station waits forthe load data to be returned and uses the next vector to pass the loadthrough the FNCU and onto the RES bus where the FIROB latches it andeventually writes it back to a the correct register. The second dispatchposition is to add 2h or 4h to ESP.

Interface for Multiply, Divide, SRB, and FPU

Each of the four issue positions has AOPND and BOPND busses which aretristated and transfer operands from the operand steering unit, RS2,RS1, and RS0 to the FNCU. No forwarding is allowed on these AOPND andBOPND busses. Forwarding from the RES busses is multiplexed directly atthe input to the FNCU. These AOPND and BOPND busses also have tristatedrivers onto the SAOPND and SBOPND busses which are shared operandbusses going to the integer multiplier block.

Floating point instructions go to the Integer uCode to have one FIROBline with four entries assigned to it. If needed, the first threedispatch positions will contain linear address calculations for the 3possible floating point loads (32 bits, 32 bits, and 16 bits), and thoseinteger reservation station entries free up after the linear addresscalculations for the FPU. The last dispatch position is a NOOP from thefunctional unit point of view. The ucode sends an Fcode entry point(corresponding to the last dispatch position) directly to the FPU. Forfloating point stores, the integer reservation stations perform thelinear address calculations, and the LSSEC latches the linear addresses.Those integer reservation station entries then free up. Later, the FPUwould request the multiplier for use of the MUL result bus and indicateto the reservation station control for position 3 that it needs to drivestore data from the FPU to the MUL result bus to the FNCU3 result MUX tothe RES3 bus. The LSSEC latches the data off the RES3 bus and performsthe stores.

The SRB instructions go to issue positions in any order. They use asimilar protocol to the LOAD/STORE instructions. The SRB contains a fourentry file which receives tags and other information from theIDECODE/DISPATCH. SRB opcodes can pass through the functional units inany order and then be reordered in the SRB file before the SRB executesthem. No renaming is done on the SRB registers (CR0, CR4, various arrayentries, etc.). Data from the SRB to a standard x86 register ortemporary register is returned to the FIROB using the LSRES0 bus and canbe forwarded to the reservation stations at the same time. The followingexample shows how information is sent to the SRB and how information isforwarded from the LSRES0 bus. The actual SRB transfers (setup and move)would take several cycles to complete and the latencies are not shown inthis example.

The SRB flow follows:

    ______________________________________                                        RES.sub.-- STA  RES.sub.-- STA                                                                DTAG=2.sub.-- 2                                                               BTAG=1.sub.-- 0                                               pass SRB info   idle        idle                                              FNCU0           FNCU2       LSSEC:SRB                                         .sub.-- RES0.sub.--                                                                           .sub.-- RES2.sub.--                                                                       .sub.-- LSRES.sub.--                              pass                                                                          DTAG=1.sub.-- 0                                                               R.sub.-- SRB                                                                  --------------------------------------------------------------------------    --------------                                                                RES.sub.-- STA  RES.sub.-- STA                                                                DTAG=2.sub.-- 2                                                               BTAG=1.sub.-- 0                                                                           perform                                           idle            idle        reg <- CR0                                        FNCU0           FNCU2       LSSEC:SRB                                         .sub.-- RES0.sub.--                                                                           .sub.-- RES2.sub.--                                                                       .sub.-- LSRES.sub.--                                                          LSTAG=1.sub.-- 0                                                              R.sub.-- SRB.sub.-- DAT                           --------------------------------------------------------------------------    --------------                                                                RES.sub.-- STA  RES.sub.-- STA                                                idle            execute     idle                                              FNCU0           FNCU2       LSSEC:SRB                                         .sub.-- RES0.sub.--                                                                           .sub.-- RES2.sub.--                                                                       .sub.-- LSRES.sub.--                                              execute                                                                       DTAG=2.sub.-- 2                                                               R.sub.-- VALID                                                ______________________________________                                    

Functional Unit Handling of Unaligned operands and SIB

When a byte operand such as AH is not located in the lowest bits, thefunctional unit needs to shift the AH operand from bits (15:8) to bits(7:0), perform the operation, and shift the result back to bits (15:8).The AHBYTE information will tell the functional unit when this alignmentis required. SIB (scale, index, and base) addressing forms are sent astwo dispatch positions because the instructions require dependencychecking and renaming on three source operands. During the first opcodethe base and the scaled index are added together, the result is given tothe second opcode to perform the actual operation. IDECODE will sendscale information to the functional unit indicating shifts by (0,1,2,3,or 4). Except for the scale information, the FNCU would not have neededto know anything about SIBs.

Integer Multiplier

The integer multiplier will use a two iteration method, the first passgoes through an 8×32 array, then through the Wallace tree, and if thefinal result is 32 bits or less, go through the carry propagate adderand be finished, else take another iteration using a larger Wallace treeto combine the carries and sums from the first iteration with thecarries and sum generated from the 8×32 array on the second iteration,and finish with a pass through the carry propagate adder.

As the reservation stations get all of the operands needed for themultiply, they will send the multiply on the SAOPND and the SBOPNDbusses which are shared tristate busses from the AOPND and the BOPND ofeach reservation station group. There will be a a total of three cyclesused for the multiply. The Multiply block and the FPU will arbitrate forthe MUL result bus such that the multiplier always has highest priority.

X86 Assembly Instruction Mapping for Fast Path

Processor 500 executes a small subset of the x86 instruction set (around50 instructions) as fast path instructions while everything else getssent to microcode. The current startup delay for the microcode engine isone extra cycle. The following chart lists the instructions which go tofast path and the ones which get sent to the microcode engine. Some ofthe x86 instructions are also broken up into two dispatch positions.

    ______________________________________                                        x86 Assembly                                                                              Fast Path  Opcode seen by FNCU                                    AAA                    uCode                                                  AAD                    uCode                                                  AAM                    uCode                                                  AAS                    uCode                                                  ADC         fast       ADC                                                    ADD         fast       ADD                                                    AND         fast       AND                                                    ARPL                   uCode                                                  BOUND                  uCode                                                  BSF         fast       BSF (may become uCode                                              eight bits at                                                                            with 16 bit BSF)                                                   a time                                                            BSR         fast       BSR (may become uCode                                              eight bits at                                                                            with 16 bit BSR)                                                   at time                                                           BSWAP                  uCode                                                  BT          fast       BT                                                     BTC         fast       BTC                                                    BTR         fast       BTR                                                    BTS         fast       BTS                                                    CALL        fast       near indirect or near                                                         relative else uCode                                    CBW         fast       SIGNXA                                                 CWDE        fast       SIGNXA                                                 CLC         fast       use flag equations                                     CLD         fast       use flag equations                                     CLI                    uCode                                                  CLTS                   uCode                                                  CMC         fast       use flag equations                                     CMP         fast       SUB (no result writeback                                                      by FIROB)                                              CMPS                   uCode                                                  CMPSB                  uCode                                                  CMPSW                  uCode                                                  CMPSD                  uCode                                                  CMPXCHG                uCode                                                  CMPXCHG8B              uCode                                                  CPUID                  uCode                                                  CWD                    uCode                                                  CWQ                    uCode                                                  DDA                    uCode                                                  DAS                    uCode                                                  DEC         fast       SUB wo/ CF save                                        DIV                    uCode                                                  ENTER                  uCode                                                  HLT                    uCode                                                  IDIV                   uCode                                                  IMUL        fast       IMUL, some are 2 dispatch                                                     pos                                                    IN                     uCode                                                  INC         fast       ADD wo/ CF save                                        INS                    uCode                                                  INSB                   uCode                                                  INSW                   uCode                                                  INSD                   uCode                                                  INT                    uCode                                                  INTO                   uCode                                                  INVD                   uCode                                                  INVLPG                 uCode                                                  IRET                   uCode                                                  IRETD                  uCode                                                  Jcc         fast       JMPCC                                                  JCXZ        fast       two dispatch positions                                 JECXZ       fast       two dispatch positions                                 JMP         fast       near and indirect else                                                        uCode                                                  LAHF        fast       LAHF                                                   LAR                    uCode                                                  LDS                    uCode                                                  LES                    uCode                                                  LFS                    uCode                                                  LGS                    uCode                                                  LSS                    uCode                                                  LEA         fast       LEAB                                                   LEAVE       fast       two dispatch position                                  LGDT                   uCode                                                  LIDT                   uCode                                                  LLDT                   uCode                                                  LMSW                   uCode                                                  LODS                   uCode                                                  LODSB                  uCode                                                  LODSW                  uCode                                                  LODSD                  uCode                                                  LOOP        fast       two dispatch positions                                 LOOPcond               uCode                                                  LSL                    uCode                                                  LTR                    uCode                                                  MOV         fast       MOVZX                                                  MOVCC       fast       MOVCC                                                  MOV CR                 uCode                                                  MOV DR                 uCode                                                  MOVS                   uCode                                                  MOVSB                  uCode                                                  MOVSW                  uCode                                                  MOVSD                  uCode                                                  MOVSX       fast       MOVSX                                                  MOVZX       fast       MOVZX                                                  MUL         fast       MUL; some are two                                                             dispatch pos                                           NEG         fast       SUB                                                    NOP                    not sent to FNCU                                       NOT         fast       SUB                                                    OR          fast       OR                                                     OUT                    uCode                                                  OUTS                   uCode                                                  OUTSB                  uCode                                                  OUTSW                  uCode                                                  OUTSD                  uCode                                                  POP         fast       two dispatch positions                                 POPA                   uCode                                                  POPAD                  uCode                                                  POPF                   uCode-may change IOPL                                  POPFD                  uCode-may change IOPL                                  PUSH        fast       some are two dispatch pos                              PUSHA                  uCode                                                  PUSHAD                 uCode                                                  PUSHF       fast       MOVF                                                   PUSHFD      fast       MOVF                                                   RCL                    uCode using RCL.sub.-- 1                               RCR                    uCode using RCR.sub.-- 1                               ROL         fast       ROL                                                    ROR         fast       ROR                                                    RDMSR                  uCode                                                  REP                    uCode                                                  REPE                   uCode                                                  REPZ                   uCode                                                  REPNE                  uCode                                                  REPNZ                  uCode                                                  RET         fast       two dispatch positions                                 RSM                    uCode                                                  SAHF        fast       SAHF                                                   SAL         fast       SHL                                                    SAR         fast       SAR                                                    SHL         fast       SHL                                                    SHR         fast       SHR                                                    SBB         fast       SBB                                                    SCAS                   uCode using DFADD                                      SCASB                  uCode using DFADD                                      SCASW                  uCode using DFADD                                      SCASD                  uCode using DFADD                                      SETcc       fast       SETCC                                                  SGDT                   uCode                                                  SIDT                   uCode                                                  SHLD                   uCode                                                  SHRD                   uCode                                                  SLDT                   uCode                                                  SMSW                   uCode                                                  STC         fast       ZEROXA & flags=BCD.sub.-- FS                           STD         fast       FIROB                                                  STI                    uCode                                                  STOS                   uCode                                                  STOSB                  uCode                                                  STOSW                  uCode                                                  STOSD                  uCode                                                  STR                    uCode                                                  SUB         fast       SUB                                                    TEST        fast       AND (no result writeback                                                      by FIROB)                                              VERR                   uCode                                                  VERW                   uCode                                                  WBINVD                 uCode                                                  WRMSR                  uCode                                                  XADD                   uCode                                                  XCHG                   uCode                                                  XLAT        fast       MOV                                                    XLATB       fast       MOV                                                    XOR         fast       XOR                                                    ______________________________________                                    

Details of the Divide Mnemonics and Their Use

The restoring division algorithm is done using ucode and FNCU0 only.Local divide direction bits are kept by FNCU0 and can be used to selectbetween two different vectors (one for ADD and one for SUB). Numeroussignals are supplied from the control bit vector to the FNCU0 speciallogic so that proper handling of the division direction bit and propermultiplexing of the result can be performed.

FNCU0 Signal List

Functional Unit 0 is the only one which can do divides. This signal listshows where the various functional unit signals come from or go to.

FNCU0 Output Signals

    ______________________________________                                        F0BRN.sub.-- ADR;                                                                            assert proper branch status                                    F0BRN.sub.-- NT;                                                                             assert proper branch status                                    F0BRN.sub.-- OK;                                                                             assert proper branch status                                    F0BRN.sub.-- T.sub.-- ADR;                                                                   assert proper branch status                                    F0BRN.sub.-- T.sub.-- OK;                                                                    assert proper branch status                                    F0STAT.sub.-- LD;                                                                            RES bus status - load linear address                           F0STAT.sub.-- MUL;                                                                           RES bus status - multiplier                                    F0STAT.sub.-- NONE;                                                                          RES bus status - no result                                     F0STAT.sub.-- OPST;                                                                          RES bus status - operand data for a                            store                                                                         F0STAT.sub.-- SRB;                                                                           RES bus status - SRB information                               F0STAT.sub.-- ST;                                                                            RES bus status - store linear address                          F0STAT.sub.-- VAL;                                                                           RES bus status - valid                                          10:0! INSLSB; LSSEC segment MUX                                               14:0! RESLA;  early result bits send to LSSEC                                DCACHE                                                                         2:0! DTAG;    destination tag                                                 2:0! RSTAT;   result status                                                   31:0! FDRES;  forwarded RES bus                                               6:0! RFLAG;   result flags                                                   ______________________________________                                    

FNCU0 Inputs from reservation station, forwarding, and multiplier

    ______________________________________                                        DCPAPRHIT;    data cache port A predicted way hit                             DCPBPRHIT;    data cache port B predicted way hit                             DCUNPAHIT;    data cache port A unpredicted way hit                           DCUNPBHIT;    data cache port B unpredicted way hit                           FNCUGO;       functional unit GO indication                                   MULXF;        value of all multiply flags                                     OPALSRES0;    used to qualify valid status; else                              mispredicted way                                                                            miss; Aop & DC port 0                                           OPALSRES1;    used to qualify valid status; else                              mispredicted way                                                                            miss; Aop & DC port 1                                           OPBLSRES0;    used to qualify valid status; else                              mispredicted way                                                                            miss; Bop & DC port 0                                           OPBLSRES1;    used to qualify valid status; else                              mispredicted way                                                                            miss; Bop & DC port 1                                           PURGE;        resets all processes in the multiplier                          RESET;        resets all processes in the multiplier                                        and GRNT.sub.-- CNT                                              1:0! LSCANFWD                                                                              LSSEC cancel of forwarded load data on                                        LSRESn                                                           10:0! INSSEG;                                                                              LSSEC segment MUX select                                         2:0! CDTAG;  current destination tag for the                                               incoming opcode                                                  31:0! AOPND; A operand                                                        31:0! BOPND; B operand                                                        31:0! DSPR;  displacement                                                     31:0! FDRES0;                                                                              forwarded result bus from position 0                             31:0! FDRES1;                                                                              forwarded result bus from position 1                             31:0! FDRES2;                                                                              forwarded result bus from position 2                             31:0! FDRES3;                                                                              forwarded result bus from position 3                             31:0! LSRES0;                                                                              LSSEC result bus 0                                               31:0! LSRES1;                                                                              LSSEC result bus 1                                               31:0! MULTRES;                                                                             multiply floating point bus                                      6:0! FLGD;   input flag data from the reservation                                          station                                                          6:0! RFLAG0; input flag data from position 0                                  6:0! RFLAG1; input flag data from position 1                                  6:0! RFLAG2; input flag data from position 2                                  6:0! RFLAG3; input flag data from position 3                                  6:0! RFLAGLS0;                                                                             input flag data from LSSEC 0                                     6:0! RFLAGLS1;                                                                             input flag data from LSSEC 1                                     7:0! FNCUMUXA;                                                                             mux control from reservation station                                          for A operand input                                              7:0! FNCUMUXB;                                                                             mux control from reservation station                                          for B operand input                                              7:0! FNCUMUXCF;                                                                            mux control from reservation station                                          for the carry flag                                               7:0! FNCUMUXSTF;                                                                           mux control from reservation station                                          for the status flags                                            ______________________________________                                    

FNCU0 Inputs for control store--shared for both vectors

    ______________________________________                                        F0CTLST.sub.-- VWR;                                                                         valid control store write next cycle                            F0DIV0;       initial divide opcode                                           F0DIV;        divide opcode                                                   F0DIVCMP;     divide opcode                                                   F0DIVL;       divide opcode                                                   F0DIVQ;       divide opcode                                                   F0DIVREM;     divide opcode                                                   F0DIVSGN;     divide opcode                                                   F0IDIVCMP;    divide opcode                                                   F0IDIVDEND0;  divide opcode                                                   F0IDIVDEND1;  divide opcode                                                   F0IDIVSOR;    divide opcode                                                   F0SELOPA;     The initial quotient is from operand A                          else from adder output                                                        F0UPDDF;      update division direction bit                                   F0UPDQ;       update division quotient                                        SELSQXOPA;    indicates that quotient gets shifted                            quotient slave instead of adder or OPA                                         3:0! F0CTLST.sub.-- WR;                                                                    indicates which control store vector to use                      7:0! F0CTLST.sub.-- USE;                                                                   indicates which control store vector to use                      7:0! FRESMUX;                                                                              select lines for MUX to RES bus                                 ______________________________________                                    

FNCU0 Inputs for control store--vector 0 ; no shifts allowed

    ______________________________________                                        AXBLAC0;      indicates if operand A or B is used for                                       linear addr calc                                                F0ADD.sub.-- F0;                                                                            addition type flag generation                                   F0BCD.sub.-- FC0;                                                                           BCD clear type flag generation                                  F0BCD.sub.-- FS0;                                                                           BCD set type flag generation                                    F0CIN0;       carry-in from the control block                                 F0CLD.sub.-- F0;                                                                            clear direction flag generation                                 F0CMC.sub.-- F0;                                                                            complement carry type flag generation                           F0COMPB0;     complement the B operand                                        F0COND.sub.-- CODE0;                                                                        evaluate condition codes                                        F0DAA.sub.-- FS0;                                                                           DAA type flag generation                                        F0FLAGCF0;    opcode requires using a version of                                            EFLAGSCF                                                        F0LOG.sub.-- F0;                                                                            logical type flag generation                                    F0MOVCC0;     MOVCC instruction                                               F0SETCC0;     SETCC instruction                                               F0MUXCIN0;    carry-in MUX                                                    F0MUXDF0;     direction flag MUX                                              F0SETCF0;     ADC uses true version of EFLAGSCF else                                        ˜EFLAGSCF                                                 F0STD.sub.-- F0;                                                                            store direction flag generation                                 F0UNC.sub.-- BRN0;                                                                          unconditional branch evaluation                                 FISTAT.sub.-- BRN0;                                                                         assert proper branch status                                     FISTAT.sub.-- LD0;                                                                          RES bus status - load linear address                            FISTAT.sub.-- MUL0;                                                                         RES bus status - mulitplier                                     FISTAT.sub.-- NONE0;                                                                        RES bus status - no result                                      FISTAT.sub.-- OPST0;                                                                        RES bus status - operand data for a                                           store                                                           FISTAT.sub.-- SRB0;                                                                         RES bus status - SRB information                                FISTAT.sub.-- ST0;                                                                          RES bus status - store linear address                           FISTAT.sub.-- VAL0;                                                                         RES bus status - valid                                          RQLAC0;       linear address calculation                                      SIGNXA0;      sign extend the A operand                                       SIGNXB0;      sign extend the B operand                                       SIGNXD0;      sign extend the D operand                                       USEOFFSET0;   use offset for PUSH, POP, or FPU lin                                          adr calc                                                        ZEROXA0;      zero extend the A operand                                       ZEROXB0;      zero extend the B operand                                       ZEROXD0;      zero extend the D operand                                        1:0! AHBYTE0;                                                                              alignment select bits0;move bits 15:8                                         to 7:0                                                           2:0! F0GIN0; generate style inputs (used here to                                           indicate type of operation)                                      3:0! F0PIN0; propagate style inputs (used here to                                          indicate type of operation)                                      3:0! OPSIZEIN0;                                                                            size / position of result (dword, word,                                       ah, al)                                                          4:0! F0SCALE;                                                                              scale for SIB: 8,4,2,1,0                                         7:0! FRESMUX0;                                                                             select lines for MUX to RES bus                                 ______________________________________                                    

FNCU0 Inputs for control store--vector 1; shifts allowed

    ______________________________________                                        AXBLAC1;     indicates if operand A or B is used for                                       linear addr calc                                                 F0ADD.sub.-- F1;                                                                           addition type flag generation                                    F0BCD.sub.-- FC1;                                                                          BCD clear type flag generation                                   F0BCD.sub.-- FS1;                                                                          BCD set type flag generation                                     F0CIN1;      carry-in from the control block                                  F0CLD.sub.-- F1;                                                                           clear direction flag generation                                  F0CMC.sub.-- F1;                                                                           complement carry type flag generation                            F0COMPB1;    complement the B operand                                         F0COND.sub.-- CODE1;                                                                       evaluate condition codes                                         F0DAA.sub.-- FS1;                                                                          DAA type flag generation                                         F0FLAGCF1;   opcode requires using a version of                                            EFLAGSCF                                                         F0LOG.sub.-- F1;                                                                           logical type flag generation                                     SHFEFLAG1;   merge Bop and all flags into result                              F0MOVCC1;    MOVCC instruction                                                F0SETCC1;    SETCC instruction                                                F0MUXCIN1;   carry-in MUX                                                     F0MUXDF1;    direction flag MUX                                               F0SETCF1;    ADC uses true version of EFLAGSCF else                                        ˜EFLAGSCF                                                  F0STD.sub.-- F1;                                                                           store direction flag generation                                  F0UNC.sub.-- BRN1;                                                                         unconditional branch evaluation                                  FISTAT.sub.-- BRN1;                                                                        assert proper branch status                                      FISTAT.sub.-- LD1;                                                                         RES bus status - load linear address                             FISTAT.sub.-- MUL1;                                                                        RES bus status - mulitplier                                      FISTAT.sub.-- NONE1;                                                                       RES bus status - no result                                       FISTAT.sub.-- OPST1;                                                                       RES bus status - operand data for a                                           store                                                            FISTAT.sub.-- SRB1;                                                                        RES bus status - SRB information                                 FISTAT.sub.-- ST1;                                                                         RES bus status - store linear address                            FISTAT.sub.-- VAL1;                                                                        RES bus status - valid                                           RQLAC1;      linear address calculation                                       SHFBCMP1;    indicates a Bit Test and Complement                              SHFBITEST1;  a select signal for any Bit Test opcode                          SHFBITOPA1;  a select signal for only the BTEST (BT)                                       opcode                                                           SHFBSF1;     select signal for Bit Scan Forward                               SHFBSR1;     select signal for Bit Scan Reverse                               SHFBSXR1;    indicates a set/reset for bit test                                            instruction                                                      SHFEFLAG1;   merge Bop and all flags into result                              SHFFLGOF1;   overwrite the OF position of Bop to                                           form result                                                      SHFFLGEF1;   overwrite the OF position of Bop to                                           form result                                                      SHFLAHF1;    load Flags into AH opcode                                        SHFLEFT1;    indicates RCL, ROL, or SHL opcode                                SHFRIGHT1;   indicates RCR, ROR, SAR, or SHR opcode                           SHFROT1;     indicates a rotate or SHL opcode                                 SHFSAHF1;    store AH into Flags opcode                                       SHFSELSF31.sub.-- 1;                                                                       set the sign flag to the shifter output                                       bit 31                                                           SHFSETCF1;   set carry flag for RCL & RCR equal to                                         EFLAGSCF                                                         SHF.sub.-- UPD.sub.-- RC1;                                                                 select for updating the local carry                                           flag                                                             SIGNXA1;     sign extend the A operand                                        SIGNXB1;     sign extend the B operand                                        SIGNXD1;     sign extend the D operand                                        USEOFFSET0;  use offset for PUSH, POP, or FPU lin                                          adr calc                                                         ZEROXA1;     zero extend the A operand                                        ZEROXB1;     zero extend the B operand                                        ZEROXD1;     zero extend the D operand                                         1:0!AHBYTE1;                                                                              alignment select bits0;move bits 15:8                                         to 7:0                                                            21:0! SHFSELPRE1;                                                                         the preshaft mux select signals                                   2:0! S0GIN1;                                                                              generate style inputs (used here to                                           indicate type of operation)                                       2:0! SHFSELZF1;                                                                           determines which groups of bit to use                                         for setting the zero flag                                         3:0! F0PIN1;                                                                              propagate style inputs (used here to                                          indicate type of operation)                                       3:0! OPSIZEIN1;                                                                           size/position of result (dword, word,                                         ah, al)                                                           3:0! SHFSELOF1;                                                                           determines which groups of bit to use                                         for setting the overflow flag                                     3:0! SHFSELOUT1;                                                                          indicates final byte alignment for the                                        shifter output                                                    7:0! FRESMUX1;                                                                            select lines for MUX to RES bus                                   7:0! SHFSELCF1;                                                                           determines which shifter bits to use to                          set the carry flag                                                             9:0! SHFSELCNT1;                                                                          mux select signals determining source                                         of shifter count value                                           ______________________________________                                    

FNCU1 to 3 Signals List

FNCU1 to FNCU3 contain the same signals as FNCU0 except for the sharedlist

FNCU1 Inputs for control store--shared for both vectors

    ______________________________________                                        F0CTLST.sub.-- VWR;                                                                           valid control store write next cycle                           7:0! F0CTLST.sub.-- USE;                                                                     indicates which control store vector                                          to use                                                         3:0! F0CTLST.sub.-- WR;                                                                      indicates which control store vector                                          to use                                                        ______________________________________                                    

CBVS Sub-Block

Fully decoded control vectors are stored in the Control Bit Vector Storesub-block. The reservation station and/or division direction flagdetermine which vector to use. As an example vector 0 and 1 could beused by the entry 2 of the reservation station. To do the linear addresscalculation for a load, entry 2 of the reservation station could selectvector 0 and perform the add. When the load data came back, entry 2 ofthe reservation station would detect the tag match and use vector 2along with the forwarded data to perform the actual operation. Onlyvector 2 can be used for shifting.

CBVS Input Signals List

    ______________________________________                                        AXBLAC0;     indicates if operand A or B is used for                                       linear addr calc                                                 AXBLAC1;     indicates if operand A or B is used for                                       linear addr calc                                                 F0ADD.sub.-- F0;                                                                           addition type flag generation                                    F0ADD.sub.-- F1;                                                                           addition type flag generation                                    F0BCD.sub.-- FC0;                                                                          BCD clear type flag generation                                   F0BCD.sub.-- FC1;                                                                          BCD clear type flag generation                                   F0BCD.sub.-- FS0;                                                                          BCD set type flag generation                                     F0BCD.sub.-- FS1;                                                                          BCD set type flag generation                                     F0CIN0;      carry-in from the control block                                  F0CIN1;      carry-in from the control block                                  F0CLD.sub.-- F0;                                                                           clear direction flag generation                                  F0CLD.sub.-- F1;                                                                           clear direction flag generation                                  F0CMC.sub.-- F0;                                                                           complement carry type flag generation                            F0CMC.sub.-- F1;                                                                           complement carry type flag generation                            F0COMPB0;    complement the B operand                                         F0COMPB1;    complement the B operand                                         F0COND.sub.-- CODE0;                                                                       evaluate condition codes                                         F0COND.sub.-- CODE1;                                                                       evaluate condition codes                                         F0CTLST.sub.-- VWR;                                                                        valid control store write next cycle                             F0DAA.sub.-- FS0;                                                                          DAA type flag generation                                         F0DAA.sub.-- FS1;                                                                          DAA type flag generation                                         F0DIV0;      initial divide opcode                                            F0DIV;       divide opcode                                                    F0DIVCMP;    divide opcode                                                    F0DIVL;      divide opcode                                                    F0DIVQ;      divide opcode                                                    F0DIVREM;    divide opcode                                                    F0DIVSGN;    divide opcode                                                    F0FLAGCF0;   opcode requires using a version of                                            EFLAGSCF                                                         F0FLAGCF1;   opcode requires using a version of                                            EFLAGSCF                                                         F0IDIVCMP;   divide opcode                                                    F0IDIVDEND0; divide opcode                                                    F0IDIVDEND1; divide opcode                                                    F0IDIVSOR;   divide opcode                                                    F0LOG.sub.-- F0;                                                                           logical type flag generation                                     F0LOG.sub.-- F1;                                                                           logical type flag generation                                     F0MUXCIN0;   carry-in MUX                                                     F0MUXCIN1;   carry-in MUX                                                     F0MUXDF0;    direction flag MUX                                               F0MUXDF1;    direction flag MUX                                               F0SELOPA;    The initial quotient is from operand A                                        else from adder output                                           F0SETCF0;    ADC uses true version of EFLAGSCF else                                        ˜EFLAGSCF                                                  F0SETCF1;    ADC uses true version of EFLAGSCF else                                        ˜EFLAGSCF                                                  F0STD.sub.-- F0;                                                                           store direction flag generation                                  F0STD.sub.-- F1;                                                                           store direction flag generation                                  F0UNC.sub.-- BRN0;                                                                         unconditional branch evaluation                                  F0UNC.sub.-- BRN1;                                                                         unconditional branch evaluation                                  F0UPDDF;     update division direction bit                                    F0UPDQ;      update division quotient                                         FISTAT.sub.-- BRN0;                                                                        assert proper branch status                                      FISTAT.sub.-- BRN1;                                                                        assert proper branch status                                      FISTAT.sub.-- LD0;                                                                         RES bus status - load linear address                             FISTAT.sub.-- LD1;                                                                         RES bus status - load linear address                             FISTAT.sub.-- MUL0;                                                                        RES but status - mulitplier                                      FISTAT.sub.-- MUL1;                                                                        RES bus status - mulitplier                                      FISTAT.sub.-- NONE0;                                                                       RES bus status - no result                                       FISTAT.sub.-- NONE1;                                                                       RES bus status - no result                                       FISTAT.sub.-- OPST0;                                                                       RES bus status - operand data for a                                           store                                                            FISTAT.sub.-- OPST1;                                                                       RES bus status - operand data for a                                           store                                                            FISTAT.sub.-- SRB0;                                                                        RES bus status - SRB information                                 FISTAT.sub.-- SRB1;                                                                        RES bus status - SRB information                                 FISTAT.sub.-- ST0;                                                                         RES bus status - store linear address                            FISTAT.sub.-- ST1;                                                                         RES bus status - store linear address                            FISTAT.sub.-- VAL0;                                                                        RES bus status - valid                                           FISTAT.sub.-- VAL1;                                                                        RES bus status - valid                                           FNCUGO;      functional unit GO indication                                    PURGE;       resets all processes                                             RESET;       resets all processes                                             RQLAC0;      linear address calculation                                       RQLAC1;      linear address calculation                                       SELSQXOPA;   indicates that quotient gets shifted                                          quotient slave instead of adder of OPA                           SHFBCMP1;    indicates a Bit Test and Complement                              SHFBITEST1;  a select signal for any Bit Test opcode                          SHFBITOPA1;  a select signal for only the BTEST (BT)                                       opcode                                                           SHFBSF1;     select signal for Bit Scan Forward                               SHFBSR1;     select signal for Bit Scan Reverse                               SHFBSXR1;    indicates a set/reset for bit test                                            instruction                                                      SHFEFLAG1;   merge Bop & all flags into result                                SHFFLGDF1;   overwrite the DF position of Bop to                                           form result                                                      SHFFLGOF1;   overwrite the OF position of Bop to                                           form result                                                      SHFLAHF1;    load Flags into AH opcode                                        SHFLEFT1;    indicates RCL, ROL, or SHL opcode                                SHFRIGHT1;   indicates RCR, ROR, SAR, or SHR opcode                           SHFROT1;     indicates a rotate or SHL opcode                                 SHFSAHF1;    store AH into Flags opcode                                       SHFSELSF31.sub.-- 1;                                                                       set the sign flag to the shifter output                                       bit 31                                                           SHFSETCF1;   set carry flag for RCL & RCR equal to                                         EFLAGSCF                                                         SHF.sub.-- UPD.sub.-- RC1;                                                                 select for updating the local carry                                           flag                                                             SIGNXA0;     sign extend the A operand                                        SIGNXA1;     sign extend the A operand                                        SIGNXB0;     sign extend the B operand                                        SIGNXB1;     sign extend the B operand                                        SIGNXD0;     sign extend the D operand                                        SIGNXD1;     sign extend the D operand                                        USEOFFSET0;  use offset for PUSH, POP, or FPU lin                                          adr calc                                                         USEOFFSET1;  use offset for PUSH, POP, or FPU lin                                          adr calc                                                         ZEROXA0;     zero extend the A operand                                        ZEROXA1;     zero extend the A operand                                        ZEROXB0;     zero extend the B operand                                        ZEROXB1;     zero extend the B operand                                        ZEROXD0;     zero extend the D operand                                        ZEROXD1;     zero extend the D operand                                         1:0! AHBYTE0;                                                                             alignment select bits0;move bits 15:8                                         to 7:0                                                            1:0! AHBYTE1;                                                                             alignment select bits0;move bits 15:8                                         to 7:0                                                            21:0! SHFSELPRE1;                                                                         the preshift mux select signals                                   2:0! F0GIN0;                                                                              generate style inputs (used here to                                           indicate type of operation)                                       2:0! F0GIN1;                                                                              generate style inputs (used here to                                           indicate type of operation)                                       2:0! SHFSELZF1;                                                                           determines which groups of bit to use                                         for setting the zero flag                                         3:0! F0CTLST.sub.-- WR;                                                                   indicates which control store vector to                                       use                                                               3:0! F0PIN0;                                                                              propagate style inputs (used here to                                          indicate type of operation)                                       3:0! F0PIN1;                                                                              propagate style inputs (used here to                                          indicate type of operation)                                       3:0! OPSIZEIN0;                                                                           size / position of result (dword, word,                                       ah, al)                                                           3:0! OPSIZEIN1;                                                                           size / position of result (dword, word,                                       ah, al)                                                           3:0! SHFSELOF1;                                                                           determines which groups of bit to use                                         for setting the overflow flag                                     3:0! SHFSELOUT1;                                                                          indicates final byte alignment for the                                        shifter output                                                    4:0! F0SCALE;                                                                             scale for SIB: 8,4,2,1,0                                          7:0! F0CTLST.sub.-- USE;                                                                  indicates which control store vector to                                       use                                                               7:0! FRESMUX0;                                                                            select lines for MUX to RES bus                                   7:0! FRESMUX1;                                                                            select lines for MUX to RES bus                                   7:0! FRESMUX;                                                                             select lines for MUX to RES bus                                   7:0! SHFSELCF1;                                                                           determines which shifter bits to use to                                       set the carry flag                                                9:0! SHFSELCNT1;                                                                          mux select signals determining source                                         of shifter count value                                           ______________________________________                                    

CBVS Output Signal List (control bit vector store)

    ______________________________________                                        AXBLAC;      indicates if operand A or B is used for                                       linear addr calc                                                 DIV0;        initial divide opcode                                            DIV:         divide opcode                                                    DIVCMP;      divide opcode                                                    DIVL;        divide opcode                                                    DIVQ;        divide opcode                                                    DIVREM;      divide opcode                                                    DIVSGN;      divide opcode                                                    F0ADD.sub.-- F;                                                                            addition type flag generation                                    F0BCD.sub.-- FC;                                                                           BCD clear type flag generation                                   F0BCD.sub.-- FS;                                                                           BCD set type flag generation                                     F0CIN;       carry-in from the control block                                  F0CLD.sub.-- F;                                                                            clear direction flag generation                                  F0CMC.sub.-- F;                                                                            complement carry type flag generation                            F0COMPB;     complement the B operand                                         F0COND.sub.-- CODE;                                                                        evaluate condition codes                                         F0DAA.sub.-- FS;                                                                           DAA type flag generation                                         F0FLAGCF;    opcode requires using a version of                               EFLAGSCF                                                                      F0LOG.sub.-- F;                                                                            logical type flag generation                                     F0MUXCIN;    carry-in MUX                                                     F0MUXDF;     direction flag MUX                                               F0SETCF;     ADC uses true version of EFLAGSCF else                           ˜EFLAGSCF                                                               F0STD.sub.-- F;                                                                            store direction flag generation                                  F0UNC.sub.-- BRN;                                                                          unconditional branch evaluation                                  FISTAT.sub.-- BRN;                                                                         assert proper branch status                                      FISTAT.sub.-- LD;                                                                          RES bus status - load linear address                             FISTAT.sub.-- MUL;                                                                         RES bus status - mulitplier                                      FISTAT.sub.-- NONE;                                                                        RES bus status - no result                                       FISTAT.sub.-- OPST;                                                                        RES bus status - operand data for a                                           store                                                            FISTAT.sub.-- SRB;                                                                         RES bus status - SRB information                                 FISTAT.sub.-- ST;                                                                          RES bus status - store linear address                            FISTAT.sub.-- VAL;                                                                         RES bus status - valid                                           IDIVCMP;     divide opcode                                                    IDIVDEND0;   divide opcode                                                    IDIVDEND1;   divide opcode                                                    IDIVSOR;     divide opcode                                                    ISELOPA;     The initial quotient is from operand A                                        else from adder output                                           ISELSQXOPA;  indicates that quotient gets shifted                                          quotient slave instead of adder or OPA                           MOVCC;       select Bop from ALU output else take Aop                         RQLAC;       linear address calculation                                       SETCC;       set bit 0 if conditions are true                                 SHFBCMP;     indicates a Bit Test and Complement                              SHFBITEST;   a select signal for any Bit Test opcode                          SHFBITOPA;   a select signal for only the BTEST (BT)                                       opcode                                                           SHFBSF;      select signal for Bit Scan Forward                               SHFBSR;      select signal for Bit Scan Reverse                               SHFBSXR;     indicates a set/reset for bit test                                            instruction                                                      SHFEFLAG;    merge Bop & all flags into result                                SHFFLGDF;    overwrite the DF position of Bop to form                                      result                                                           SHFFLGOF;    overwrite the OF position of Bop to form                                      result                                                           SHFLAHF;     load Flags into AH opcode                                        SHFLEFT;     indicates RCL, ROL, or SHL opcode                                SHFRIGHT;    indicates RCR, ROR, SAR, or SHR opcode                           SHFROT;      indicates a rotate or SHL opcode                                 SHFSAHF;     store AH into Flags opcode                                       SHFSELSF31;  set the sign flag to the shifter output                                       bit 31                                                           SHFSETCF;    set carry flag for RCL & RCR equal to                            EFLAGSCF                                                                      SHF.sub.-- UPD.sub.-- RC;                                                                  select for updating the local carry flag                         SIGNXA;      sign extend the A operand                                        SIGNXB;      sign extend the B operand                                        SIGNXD;      sign extend the D operand                                        UPDDF;       update division direction bit                                    UPDQ;        update division quotient                                         USEOFFSET;   use offset for PUSH, POP, or FPU lin adr                                      calc                                                             ZEROXA;      zero extend the A operand                                        ZEROXB;      zero extend the B operand                                        ZEROXD;      zero extend the D operand                                         1:0! AHBYTE;                                                                              alignment select bits0; move bits 15:8 to                                     7:0                                                               21:0! SHFSELPRE;                                                                          the preshift mux select signals                                   2:0! F0GIN; generate style inputs (used here to                                           indicate type of operation)                                       2:0! SHFSELZF;                                                                            determines which groups of bit to use                                         for setting the zero flag                                         3:0! F0PIN; propagate style inputs (used here to                                          indicate type of operation)                                       3:0! OPSIZEIN;                                                                            size/position of result (dword, word,                                         ah, al)                                                           3:0! SHFSELOF;                                                                            determines which groups of bit to use                                         for setting the overflow flag                                     3:0! SHFSELOUT;                                                                           indicates final byte alignment for the                                        shifter output                                                    7:0! FRESMUX;                                                                             select lines for MUX to RES bus                                   7:0! SHFSELCF;                                                                            determines which shifter bits to use to                                       set the carry flag                                                9:0! SHFSELCNT;                                                                           mux select signals determining source of                                      shifter count value                                              ______________________________________                                    

OPSEL Sub-Block

The operand selection sub-block multiplexes the correct data in forforwarding and latches in the A operand, Boperand,Displacement/immediate bus, and flags. Sign extension and alignment forAhbyte data is also performed here.

OPSEL Input Signals (operand select and alignment)

    ______________________________________                                        AXBLAC;       indicates if operand A or B is used for                                       linear addr calc                                                F0COMPB;      complement the B operand                                        RQLAC;        linear address calculation                                      SIGNXA;       sign extend the A operand                                       SIGNXB;       sign extend the B operand                                       SIGNXD;       sign extend the D operand                                       USEOFFSET;    use offset for PUSH, POP, or FPU lin                                          adr calc                                                        ZEROXA;       zero extend the A operand                                       ZEROXB;       zero extend the B operand                                       ZEROXD;       zero extend the D operand                                        1:0! AHBYTE; alignment select bits0; move bits 15:8                                        to 7:0                                                           31:0! AOPND; A operand                                                        31:0! BOPND; B operand                                                        31:0! DSPR;  displacement                                                     31:0! FDRES0;                                                                              forwarded result bus from position 0                             31:0! FDRES1;                                                                              forwarded result bus from position 1                             31:0! FDRES2;                                                                              forwarded result bus from position 2                             31:0! FDRES3;                                                                              forwarded result bus from position 3                             31:0! LSRES0;                                                                              LSSEC result bus 0                                               31:0! LSRES1;                                                                              LSSEC result bus 1                                               3:0! OFFSET; offset used for PUSH, POP, and FPU lin                                        adr calc                                                         3:0! OPSIZEIN;                                                                             size/position of result (dword, word,                                         ah, al)                                                          4:0! F0SCALE;                                                                              scale for SIB: 8,4,2,1,0                                         6:0! FLGD;   input flag data from the reservation                                          station                                                          6:0! RFLAG0; input flag data from position 0                                  6:0! RFLAG1; input flag data from position 1                                  6:0! RFLAG2; input flag data from position 2                                  6:0! RFLAG3; input flag data from position 3                                  6:0! RFLAGLS0;                                                                             input flag data from LSSEC 0                                     6:0! RFLAGLS1;                                                                             input flag data from LSSEC 1                                     7:0! FNCUMUXA;                                                                             mux control from reservation station                                          for A operand input                                              7:0! FNCUMUXB;                                                                             mux control from reservation station                                          for B operand input                                              7:0! FNCUMUXCF;                                                                            mux control from reservation station                                          for the carry flag                                               7:0! FNCUMUXSTF;                                                                           mux control from reservation station                                          for the status flag                                             ______________________________________                                    

OPSEL Output Signals (operand select and alignment)

    ______________________________________                                         31:0! OPA;        aligned operand A                                           31:0! OPB;        aligned operand B                                           6:0! IFLG;        latched input flags                                        ______________________________________                                    

ALU Sub-Block

All logical, arithmetic, and basic moves are performed by this block.Linear address calculations and branch address calculations are alsoperformed here. Only FNCU0 is capable of performing division operations.

ALU Input Signals for FNCU0 Type

    ______________________________________                                        DIV0;          initial divide opcode                                          DIV;           divide opcode                                                  DIVCMP;        divide opcode                                                  DIVL;          divide opcode                                                  DIVQ;          divide opcode                                                  DIVREM;        divide opcode                                                  DIVSGN;        divide opcode                                                  F0CIN;         carry-in from the control block                                F0MUXCIN;      carry-in MUX                                                   F0MUXDF;       direction flag MUX                                             FNCUGO;        functional unit GO indication                                  IDIVCMP;       divide opcode                                                  IDIVDEND0;     divide opcode                                                  IDIVDEND1;     divide opcode                                                  IDIVSOR;       divide opcode                                                  ISELOPA;       The initial quotient is from operand A                                        else from adder output                                         ISELSQXOPA;    indicates that quotient gets shifted                                          quotient slave instead of adder or OPA                         PURGE;         resets all processes                                           RESET;         resets all processes                                           UPDDF;         update division direction bit                                  UPDQ;          update division quotient                                        2:0! F0GIN;   generate style inputs (used here to                                           indicate type of operation)                                     31:0! OPA;    aligned operand A                                               31:0! OPB;    aligned operand B                                               3:0! F0PIN;   propagate style inputs (used here to                                          indicate type of operation)                                     3:0! OPSIZEIN;                                                                              size / position of result (dword, word,                                       ah, al)                                                        ______________________________________                                    

ALU Input Signals for FNCU1 to FNCU3 Types

    ______________________________________                                        F0CIN;         carry-in from the control block                                F0MUXCIN;      carry-in MUX                                                   F0MUXDF;       direction flag MUX                                             FNCUGO;        functional unit GO indication                                  PURGE;         resets all processes                                           RESET;         resets all processes                                            2:0! F0GIN;   generate style inputs (used here to                                           indicate type of operation)                                     31:0! OPA;    aligned operand A                                               31:0! OPB;    aligned operand B                                               3:0! F0PIN;   propagate style inputs (used here to                                          indicate type of operation)                                     3:0! OPSIZEIN;                                                                              size / position of result (dword, word,                                       ah, al)                                                        ______________________________________                                    

ALU Output Signals

    ______________________________________                                        A0COUT14;           ALU output bit                                            A0COUT30;           ALU output bit                                            A0COUT3;            ALU output bit                                            A0COUT6;            ALU output bit                                            F0COUT15;           ALU output bit                                            F0COUT31;           ALU output bit                                            F0COUT7;            ALU output bit                                             31:0! ARES;        ALU result                                                ______________________________________                                    

SHFT Sub-Block

The shifter sub-block performs right shifts only. It can also performbit test and set/clear/complement as well as RCL by one or RCR by one.

SHFT Input Signals (shifter)

    ______________________________________                                        FNCUGO;      functional unit GO indication                                    PURGE;       resets all processes                                             RESET;       resets all processes                                             SFTCFIN;     carry in flag for shifter                                        SHFBCMP;     indicates a Bit Test and Complement                              SHFBITEST;   a select signal for any Bit Test opcode                          SHFBITOPA;   a select signal for only the BTEST (BT)                                       opcode                                                           SHFBSF;      select signal for Bit Scan Forward                               SHFBSR;      select signal for Bit Scan Reverse                               SHFBSXR;     indicates a set/reset for bit test                                            instruction                                                      SHFEFLAG;    merge Bop & all flags into result                                SHFFLGDF;    overwrite the DF position of Bop to                                           form result                                                      SHFFLGOF;    overwrite the OF position of Bop to                                           form result                                                      SHFLAHF;     load Flags into AH opcode                                        SHFLEFT;     indicates RCL, ROL, or SHL opcode                                SHFRIGHT;    indicates RCR, ROR, SAR, or SHR opcode                           SHFROT;      indicates a rotate or SHL opcode                                 SHFSAHF;     store AH into Flags opcode                                       SHFSELSF31;  set the sign flag to the shifter output                                       bit 31                                                           SHFSETCF;    set carry flag for RCL & RCR equal to                                         EFLAGSCF                                                         SHF.sub.-- UPD.sub.-- RC;                                                                  select for updating the local carry                                           flag                                                              21:0! SHFSELPRE;                                                                          the preshift mux select signals                                   2:0! SHFSELZF;                                                                            determines which groups of bit to use                                         for setting the zero flag                                         31:0! OPA;  aligned operand A                                                 31:0! OPB;  aligned operand B                                                 3:0! OPSIZEIN;                                                                            size / position of result (dword, word,                                       ah, al)                                                           3:0! SHFSELOF;                                                                            determines which groups of bit to use                                         for setting the overflow flag                                     3:0! SHFSELOUT;                                                                           indicates final byte alignment for the                                        shifter output                                                    7:0! SHFSELCF;                                                                            determines which shifter bits to use to                                       set the carry flag                                                9:0! SHFSELCNT;                                                                           mux select signals determining source                                         of shifter count value                                           ______________________________________                                    

SHFT Output Signals (shifter)

    ______________________________________                                         31:0! SHFTOUT;                                                                              shifter information for flag generation                         31:0! SRES;   shifter result                                                 ______________________________________                                    

MXDRV Sub-Block

This block multiplexes results from several different sub-blocks andfrom the multiplier and aligns the output properly before driving theresult onto the FDRES bus.

MXDRV Input Signals (output multiplier, aligment, and drivers)

    ______________________________________                                        BRN.sub.-- ADR;                                                                             assert proper branch status                                     BRN.sub.-- NT;                                                                              assert proper branch status                                     BRN.sub.-- OK;                                                                              assert proper branch status                                     BRN.sub.-- T.sub.-- ADR;                                                                    assert proper branch status                                     BRN.sub.-- T.sub.-- OK;                                                                     assert proper branch status                                     DCPAPRHIT;    data cache port A predicted way hit                             DCPBPRHIT;    data cache port B predicted way hit                             DCUNPAHIT;    data cache port A unpredicted way hit                           DCUNPBHIT;    data cache port B unpredicted way hit                           FISTAT.sub.-- BRN;                                                                          assert proper branch status                                     FISTAT.sub.-- LD;                                                                           RES bus status - load linear address                            FISTAT.sub.-- MUL;                                                                          RES bus status - multiplier                                     FISTAT.sub.-- NONE;                                                                         RES bus status - no result                                      FISTAT.sub.-- OPST;                                                                         RES bus status - operand data for a                                           store                                                           FISTAT.sub.-- SRB;                                                                          RES bus status - SRB information                                FISTAT.sub.-- ST;                                                                           RES bus status - store linear address                           FISTAT.sub.-- VAL;                                                                          RES bus status - valid                                          FNCUGO;       functional unit GO indication                                   MOVCCSEL;     select Bop from ALU input else take Aop                         OPALSRES0;    used to qualify valid status; else                                            mispredicted way miss; Aop & DC port 0                          OPALSRES1;    used to qualify valid status; else                                            mispredicted way miss; Aop & DC port 1                          OPBLSRES0;    used to qualify valid status; else                                            mispredicted way miss; Bop & DC port 0                          OPBLSRES1;    used to qualify valid status; else                                            mispredicted way miss; Bop & DC port 1                          PURGE;        resets all processes                                            RESET;        resets all processes                                             10:0! INSSEG;                                                                              LSSEC segment MUX select                                         1:0! LSCANFWD;                                                                             LSSEC cancel signal for forwarded data                                        on the LSRESn bus                                                2:0! CDTAG;  current destination tag for the                                               incoming opcode                                                  31:0! ARES;  ALU result                                                       31:0! FLGRES;                                                                              combined EFLAGS result                                           31:0! MULTRES;                                                                             multiply floating point bus                                      31:0! SRES;  shifter result                                                   3:0! OPSIZEIN;                                                                             size / position of result (dword, word,                                       ah, al)                                                          7:0! FRESMUX;                                                                              select lines for MUX to RES bus                                 ______________________________________                                    

MXDRV Output Signals (output multiplier, aligment, and drivers)

    ______________________________________                                        F0BRN.sub.-- ADR;                                                                            assert proper branch status                                    F0BRN.sub.-- NT;                                                                             assert proper branch status                                    F0BRN.sub.-- OK;                                                                             assert proper branch status                                    F0BRN.sub.-- T.sub.-- ADR;                                                                   assert proper branch status                                    F0BRN.sub.-- T.sub.-- OK;                                                                    assert proper branch status                                    F0STAT.sub.-- LD;                                                                            RES bus status - load linear address                           F0STAT.sub.-- MUL;                                                                           RES bus status - multiplier                                    F0STAT.sub.-- NONE;                                                                          RES bus status - no result                                     F0STAT.sub.-- OPST;                                                                          RES bus status - operand data for a                                           store                                                          F0STAT.sub.-- SRB;                                                                           RES bus status - SRB information                               F0STAT.sub.-- ST;                                                                            RES bus status - store linear address                          F0STAT.sub.-- VAL;                                                                           RES bus status - valid                                          10:0! INSLSB; LSSEC segment MUX                                               14:0! RESLA;  early result bits send to LSSEC                                               DCACHE                                                          2:0! DTAG;    destination tag                                                 2:0! RSTAT;   result status                                                   31:0! FDRES;  forwarded RES bus                                              ______________________________________                                    

FLGCC Sub-Block

This block is responsible for the flag generation, condition codeevaluation, and branch evaluation. The CF, carry flag, is generated thesame same and can be immediately forwarded with the result data.However, the other six flags are generated next cycle and can only beforwarded to such operations as jumps which can accept the flagsmid-cycle.

FLGCC Input Signals (flags generation, condition codes and branchevaluation)

    ______________________________________                                        A0COUT14;     ALU output bit                                                  A0COUT30;     ALU output bit                                                  A0COUT3;      ALU output bit                                                  A0COUT6;      ALU output bit                                                  F0ADD.sub.-- F;                                                                             addition type flag generation                                   F0BCD.sub.-- FC;                                                                            BCD clear type flag generation                                  F0BCD.sub.-- FS;                                                                            BCD set type flag generation                                    F0CLD.sub.-- F;                                                                             clear direction flag generation                                 F0CMC.sub.-- F;                                                                             complement carry type flag generation                           F0COND.sub.-- CODE;                                                                         evaluate condition codes                                        F0COUT15;     ALU output bit                                                  F0COUT31;     ALU output bit                                                  F0COUT7;      ALU output bit                                                  F0DAA.sub.-- FS;                                                                            DAA type flag generation                                        F0LOG.sub.-- F;                                                                             logical type flag generation                                    F0STD.sub.-- F;                                                                             store direction flag generation                                 F0UNC.sub.-- BRN;                                                                           unconditional branch evaluation                                 FISTAT.sub.-- BRN;                                                                          assert proper branch status                                     FNCUGO;       functional unit GO indication                                   MOVCC;        select Bop from ALU output else take                                          Aop                                                             MULXF;        value of all multiply flags                                     PURGE;        resets all processes                                            RESET;        resets all processes                                            SETCC;        set bit 0 if conditions are true                                 31:0! OPB;   aligned operand B                                                31:0! SHFTOUT ;                                                                            shifter information for flag generation                          3:0! OPSIZEIN;                                                                             size / position of result (dword, word,                                       ah, al)                                                          6:0! IFLG;   latched input flags                                             ______________________________________                                    

FLGCC Output Signals (flags generation, condition codes and branchevaluation)

    ______________________________________                                        BRN.sub.-- ADR;                                                                            assert proper branch status                                      BRN.sub.-- NT;                                                                             assert proper branch status                                      BRN.sub.-- OK;                                                                             assert proper branch status                                      BRN.sub.-- T.sub.-- ADR;                                                                   assert proper branch status                                      BRN.sub.-- T.sub.-- OK;                                                                    assert proper branch status                                      MOVCCSEL;    select Bop from ALU output else take Aop                          31:0! FLGRES;                                                                             combined EFLAGS result                                            6:0! RFLAG; result flags                                                     ______________________________________                                    

Multiplier Signal List

Multiplier Input Signals List

    ______________________________________                                        MULCYC2X3;      number of multiply cycles is 2,                                               else use 3                                                    MULONEOP;       opcode is for the one operand                                                 version                                                       MULRQ0;         multiply request from position 0                              MULRQ1;         multiply request from position 1                              MULRQ2;         multiply request from position 2                              MULRQ3;         multiply request from position 3                              MULSIGN;        signed values                                                 PURGE;          resets all processes in the                                                   multiplier                                                    RESET;          resets all processes in the                                                   multiplier and GRNT.sub.-- CNT                                 1:0! MULOPSIZE;                                                                              input operands are 01: byte, 10:                                              word, or 11: dword                                             31:0! SAOPND;  shared A operand bus                                           31:0! SBOPND;  shared B operand bus                                          ______________________________________                                    

Multiplier Outputs

    ______________________________________                                        MULGR0;         multiply grant to position 0                                  MULGR1;         multiply grant to position 1                                  MULGR2;         multiply grant to position 2                                  MULGR3;         multiply grant to position 3                                  MULXF;          value of all multiply flags                                    31:0! MULRES;  multiplier output bus 0                                       ______________________________________                                    

Overview of the Processor 500 FIROB

This section describes the Four-Issue Re-Order Buffer (FIROB), includingmethods of reducing the dependency checking time. The Processor 500FIROB has a structure including the data array, the status and controlarrays with associated control logic, and the special registers. Thefour-issue ROB is line-oriented which means that the number ofdispatched instructions is always 4 (some of the instructions may beNOOP), and the number of retired instructions is always 4. The FIROBsize is 6 lines of 4 instructions for a total of 24 entries. There are afew exceptions to retire the partial line such as branch mis-prediction,interrupt, and re-synchronization. There are 8 read buses and 4 retirebuses to support a line of Processor 500 instructions. The number ofresult buses are 6, 4 result buses are for results from 4 functionalunits and 2 results buses are for load/store and special registerinstructions to return data to the FIROB. The buses are 32-bits wide.The FIROB supports a massive number of comparators to dispatch 4instructions in a line. To support the push/pop instructions withindirect references to the ESP, the FIROB includes 6 ESP latches. Thelatches are strictly for updating the ESP in the register filecorrectly.

Top Level of the FIROB

The FIROB is accessed by a line of instructions instead of an individualinstruction. The line has 4 instructions, some instructions can be NOOP.Line-oriention has the advantage of a single input and allocation in theFIROB. The logic to deal with one line is much simpler than logic todeal with a combination of 0 to 4 instructions. Since the clock cycletime is short, a new method has to be implemented to do the dependencychecking in one clock cycle and driving the data in the next cycle. TheFIROB includes of 6 lines of instructions, where each line has 4instructions. FIROB will have one clock cycle to compare the readaddresses of the dispatched line to the destination entries of theprevious 5 lines for dependency and to check the dependency within thedispatched line. The FIROB implements new status bits to indicate themost up-to-date destination to reduce the dependency checking time. TheX86 instructions allow 2 operands, wherein one of the operands can beboth destination and source. The FIROB checks dependency on both sourceand destination operands, the dependency of the destinations sets thelast-in-line status which reduces the future dependency checking. A setof ESP latches are implemented to provide fast execution of the push/popinstructions. The FIROB can keep track and generate dependency data forthe ESP without stalling or serializing any ESP instructions.

Organization of the FIROB

Processor 500 uses fixed issue positions for the decode units and thefunctional units. The FIROB and the register file conform to thisarrangement. The operand addresses from the decode unit and the resultbuses from the functional unit individually access the fixed positionsof the FIROB. The proposed arrangement of the FIROB is to have theaddress and data registers, the comparator, and the control status bitsfor the comparator in the data path between the decode units and thefunctional units. Other status bits and control logic are on one side ofthe data path as shown in FIG. 33. A proposed layout of the FIROB andregister file is described below. The read buses can come from any entryin the FIROB because of forwarding. An operand bus steering may beneeded to take the inputs from the FIROB or REGF and route to thefunctional units.

FIROB is organized as 6 lines of 4 instructions each. The pointer to theentries has two parts: a 3-bit line pointer and a 2-bit entry pointer.The line pointer increases after every dispatch, and the whole line isdispatched or retired at one time. This is 3-bit incrementor and wrapsaround at the count of 5. Other reorder buffers (ROBs) are implementedwith read and write pointers, the read pointer increases as instructionsare dispatched and the write pointer increases as the instruction areretired. Since Processor 500 FIROB allocates or retires one line ofinstructions at a time and the number of dependency comparator is large,FIROB should be a FIFO (First-In-First-Out) instead of moving thepointers. The dependency comparators are always at lines 0-4. Nodependency checking is needed in line 5, and the retire line (writepointer) is always from line 0. As a line of instructions is retiredfrom line 0, lines 1-5 will shift up by 1. The read pointer increases asa new line is dispatched and decreases as a line is retired. To trackthe FIROB line number for the instructions in the reservation station,functional units, and load/store section, a virtual line pointer isassigned to each line as the line is dispatched from the decode units.The virtual line pointer is used by the reservation stations for resultforwarding, by the functional unit and the load/store section to returnresult data to the FIROB, and by the load/store buffer to access thedata cache in program order. The FIROB use the virtual line pointer tolatch the result data. The virtual line pointer is attached to each linein the FIROB and circular shifted as the line is retired from the FIROB.

From FIG. 33, the result data of the FIROB and the register file drivethe source data to 8 horizontal buses. Each functional unit receives 2read buses from these horizontal buses. The layout of the result data ofthe FIROB and the register file sis adjacent to access the horizontalbuses directly. A suggested layout organization is illustrated in FIG.34.

Signal list

IRESET--Global signal used to reset all decode units. Clears all states.

NMI₋₋ P--Input from BIU indicates non-maskable interrupt, the FIROBgenerates a clean instruction boundary trap to a fixed entry point. TheFIROB is sensitive only to the rising edge of this signal

INTR₋₋ P--Input from BIU indicates the external interrupt. This signalis qualified with the IF bit of the EFLAGS register. The interruptoccurs at appropriate instruction boundaries.

SRBHALT--Input from SRB to enter HALT mode. The FIROB stops retiringinstructions until RESET, NMI, or external interrupt occurs. The FIROBmust retire the HALT instruction before shutting down.

CR0NE--Input from SRB indicates the NE bit of the CR0 register. The NEbit indicates the floating point exception can be trapped directly(NE=1) or via XFERR₋₋ P and an external interrupt (NE=0). XIGNNE₋₋P--Input from BIU indicates the copy of pin IGNNE. When CRONE=0, thissignal is inspected to response to enabled floating point exceptions.

XFLUSH₋₋ P--Input from BIU indicates an external flush request occurs.It is falling edge sensitive and trap on instruction boundary. It issample during IRESET to enter tri-state test mode, the FIROB should notgenerate exception.

IINIT--Input from BIU indicates an initialization request. It is risingedge sensitive and trap on instruction boundary. It is sample duringIRESET to enter BIST test mode, the FIROB generates on of the two resetentry point.

MVTOSRIAD--Input from SRB, indicates a move to IAD special register,FIROB needs to check its pointer against the pointer driven on IAD.

MVFRSRIAD--Input from SRB, indicates a move from IAD special register,FIROB needs to check its pointer against the pointer driven on IAD.

MVTOARIAD--Input from SRB, indicates a move to IAD special registerarray, FIROB needs to check its pointer against the pointer driven onIAD.

MVFRARIAD--Input from SRB, indicates a move from IAD special registerarray, FIROB needs to check its pointer against the pointer driven onIAD.

MROMDEC(3:0)--Input from MROM indicates the microcodes are being decodedby the decode units. Use to set the ROBEXIT bit.

RESx(31:0)--Input from FU indicates result data. DTAGx(2:0)--Input fromFU indicates FIROB line number of the result.

DSTATx(3:0)--Input from FU indicates the status of the result data:

0000--no result

0001--valid result

0010--valid result, shift by zero

0011--exception with vector

0100--software interrupt with vector

0101--TLB miss with vector

0110--load/store breakpoint

0111--exchange result

1000--exchange with underflow

1001--exchange abort

1010--branch taken, mis-prediction

1011--branch not taken, mis-prediction

1100--reserved for FPU

1101--reserved for FPU

1110--reserved for FPU

1111--reserved for FPU

RFLAGx(31:0)--Input from FU indicates result flags.

LSTAG0(4:0)--Input from LSSEC indicates FIROB line number of the firstaccess.

LSTAG1(4:0)--Input from LSSEC indicates FIROB line number of the secondaccess.

LSRES0(31:0)--Input from LSSEC indicates result data of the firstaccess.

LSRES1(31:0)--Input from LSSEC indicates result data of the secondaccess.

IDPC1(31:0)--Input from Idecode indicates the current line PC of thefirst instruction in the 4 dispatched instructions.

IDPC2(31:0)--Input from Idecode indicates the current line PC of asecond instruction which cross the 16-byte boundary or branch target inthe 4 dispatched instructions.

ICPOSx(3:0)--ICLK7 Input from Icache to decode units indicates the PC'sbyte position of the instruction.

IDxDAT(1:0)--Input from Idecode indicates the data size information.01-byte, 10-half word, 11-word, 00-not use.

IDXADDR--Input from Idecode indicates the address size information. 1-32bit, 0-16 bit.

ICVALI(3:0)--Input from Icache indicates valid instructions. NOOP isgenerated for invalid instruction.

NODEST(3:0)--Input from Icache indicates no destination for the firstrop of the SIB-byte instruction.

DEPTAG(3:1)--Input from Icache indicates forced dependency tag on thefirst instruction; the second rop of the SIB-byte instruction.

DxUSEFL (2:0)

DxWRFL(2:0)--Input from Icache indicates the type of flag uses/writesfor this instruction of decode units:

xx1 CF-carry flag,

x1x OF-overflow flag,

1xx SF-sign, ZF-zero, PF-parity, and AF-auxiliary carry

DxUSE1(1:0)--Input from Icache indicates the type of operand being senton operand 1 for decode units. Bit 1 indicates source operand, and bit 0indicates destination operand.

DxUSE2--Input from Idecode indicates indicates source operand.

INSDISP(3:0)--Input from Idecode indicates that the instruction indecode unit is valid, if invalid, NOOP is passed to FIROB.

RDxPTR1(5:0)--Input from Icache indicates the register address foroperand 1 of the instructions.

RDxPTR2(5:0)--Input from Icache indicates the register address foroperand 2 of the instructions.

INSLSxB(5:0)--Input from decode units indicates the prefix values. bit5--data size, bit 4--address size, bit 3--lock, bit 2:0--segmentregisters.

IDECJAMIC--Output indicates that an interrupt or trap is being taken.Effect on Icache is to clear all pre-fetch or access in progress, andset all state machines to Idle/Reset.

EXCEPTION--Global output indicates that an interrupt or trap is beingtaken including resynchronization. Effect on Idecode and Fus is to clearall instructions in progress.

REQTRAP--Global output, one cycle after EXCEPTION, indicates that thetrap is initiated with new entry point or new PC is driven.

SYNC--Output indicates whether the new entry point or new PC is driven.

EXCHGSYNC--Output indicates exchange instruction resynchronization toIcache. This occurs when an exchange with a masked underflow is retired.It is a special resynchronize exchange with alternate entry point.

XFERR₋₋ P--Output to BIU indicates the floating point error which isinverted of the ES bit from the slave of the floating point statusregister. It is also used by the FIROB to generate the plunger traps.

EFLAGSAC

EFLAGSVM

EFLAGSRF

EFIOPL(13:12)

EFLAGSOF

EFLAGSDF

EFLAGSAF

EFLAGSCF--Output generates from the EFLAGS register, these bits arevisible from the slave copy of the EFLAGS register. The RF bit is alsoused in the FIROB to handle instruction breakpoint.

BRNMISP--Input from the Branch execution of the FU indicates that abranch mis-prediction. The Idecode clears all instructions in progress.

UPDFPC--Output to Icache indicate that a new Fetch PC has been detected.This signal accompanies the FPC for the Icache to begin access the cachearrays.

TARGET(31:0)--Output to Icache as the new PC for branch correction path.

BRNMISP--Input to Icache indicates that a branch mis-prediction. TheIcache changes its state machine to access a new PC and clears allpending instructions.

BRNTAKEN--Output to Icache indicates the status of the mis-prediction.This signal must be gated with UPDFPC.

BRNFIRST--Output to Icache indicates the first or second target in theICNXTBLK for updating the branch prediction.

BRNCOL(3:0)--Output to Icache indicates the instruction byte forupdating the branch prediction in the ICNXTBLK.

FPCTYP--Input to Icache indicates the type of address that is beingpassed to the Icache.

BPC(11:0)--Output indicates the PC index and byte-pointer of the branchinstruction which has been mis-predicted for updating the ICNXTBLK.

ROBEMPTY--Output indicates the FIROB is empty.

ROBFULL--Output indicates the FIROB is full.

LINEPTR(2:0)--Output indicates the current line pointer in the FIROB forthe dispatch line of instructions.

WBLPTR(2:0)--Output indicates the write-back line pointer in the FIROBfor the retiring line of instructions.

WBxNC--Output indicates the invalid write-back data to the register filefor retiring instructions.

WBxPTR(5:0)--Output indicates the write-back pointer to the registerfile for retiring instructions.

WBxD(31:0)--Output indicates the write-back data to the register filefor retiring instructions.

WBxBYTE(3:0)--Output indicates the write-back selected bytes to theregister for retiring instructions.

RBxDAT1(31:0)--Output indicates the first source operand data fordispatching instructions.

RBxDAT2(31:0)--Output indicates the second source operand data fordispatching instructions.

FLGxDAT1(5:0)--Output indicates the status flags for dispatchinginstructions.

RBxTAG1(4:0)--Output indicates the first dependency tag for dispatchinginstructions.

RBxTAG2(4:0)--Output indicates the second dependency tag for dispatchinginstructions.

FCFxTAG(4:0)--Output indicates the CF flag dependency tag fordispatching instructions.

FOFxTAG(4:0)--Output indicates the CF flag dependency tag fordispatching instructions.

FXFxTAG(4:0)--Output indicates the CF flag dependency tag fordispatching instructions.

PUSHPOP(2:1)--Output to register file indicates the pop bits of thefloating point status register to clear the full bits of the registerbeing popped. FPTOP(2:0) contains the current top-of-stack when thesebits are asserted.

FPTOP(2:0)--Output to register file indicates the current top-of-stackto identify the registers being popped to clear the full bits.

WBEXCHG--Output to register file indicates the exchange instructionbeing retired. It causes the permanent remapping register to be updatedfrom the write-back bus.

WRPTR(4:0)--Output to LSSEC indicates the bottom (oldest) entry in theFIROB without valid result. If this entry matches the store or load-missentry in the LSSEC, the entry can access the data cache at this time.

CANENTRY--Output to LSSEC indicates the bottom entry in the FIROBwithout valid result is canceled. If this entry matches the store orload-miss entry in the LSSEC, the entry can return without access thedata cache at this time.

WRPTR1(4:0)--Output to LSSEC indicates the next to bottom entry in theFIROB without valid result. If this entry matches the store or load-missentry in the LSSEC, the entry can access the data cache.

CANENTRY--Output to LSSEC indicates the next to bottom entry in theFIROB without valid result is canceled. If this entry matches the storeor load-miss entry in the LSSEC, the entry can return without access thedata cache.

Basic Operations

The FIROB interfaces with the decode units for dispatching instructions,with the functional units and LSSEC for results, and with the registerfile for retiring instructions. The FIROB updates the special registerscorrectly with each retiring instructions, handles trap/interruptgracefully, and re-synchronizes the pipeline after branchmis-prediction, self-modifying code, or changing the code segmentregister.

Dispatch Interface

Every cycle, the FIROB indicates the space status to the decode unit viaROBFULL or ROBEMPTY. As long as the FIROB is not full a line ofinstructions can be dispatched. The empty status is for serializedinstructions. The dependency checking is performed for the operands andflags of the top 5 valid lines of the FIROB. The FIROB performs thedependency checking for 2 register operands and validates the resultswith decode information and hit signals. The FIROB keeps the destinationoperands of the X86 instructions.

The FIROB provides the dependency tags for the source operands andflags. The destination tags is by the virtual line number. The fixedlocation of the instructions is simple for returning of the results. TheFIROB implements a FIFO shifter to limit the dependency checking to thefirst 5 lines. The virtual line number is shifted along with retiringline to keep track of the instructions in the functional units.

Result Bus Interface

The result buses are dedicated between the functional units and thedestination buffers of the FIROB. The FIROB compares the line number forreturning results. The exceptions are the floating point unit and LSSEC.The line number and the entry number are compared in this case. Thefunctional units send the data and address to the LSSEC for data cacheaccess. The result buses contain the status flags, and results. Variousstatus/control bits in the FIROB will be set and cleared for the benefitof the retire logic. Since the store and move-to-special-registerinstructions must access the data cache in the program order, allprevious instructions must be completed with valid results. The FIROBbroadcasts the top two entries which do not have the valid results tothe LSSEC and SRB. If the broadcasted entries match the store orload-miss instructions, the LSSEC can access the data cache and returnsresults. Similar procedure is used for the move-to-special-registerinstruction in the SRB.

The result data and tag are returned to the FIROB in the execution anddata cache access cycle. The status can be a cycle later because of thezero-detect, carry, and tag-hit in the data cache. The retire logic maynot be able to retire the instructions with late result immediately(require some work in logic and circuit design). There may be a cycledelay in retiring instructions with late statuses. Furthermore, thelimit violation checking on data access in the LSSEC can be later in thenext clock cycle along with the late tag-hit status. The limit violationresults in trap instead of retiring the instruction.

Write-Back Interface

The write-back interface of the FIROB is to ensure the retiring ofinstructions in program order. In addition to controlling write back tothe register file, the FIROB updates the EIP register and flagsregisters and controls the order of the load-miss, store accesses andmove-to-special-register instructions. In normal operation, the FIROBretires an entire line of instructions at a time. All instructions inthe line must have the valid results from the functional units or LSSEC.All entries with valid result destination will write back to theregister file. The LIL and NC status bits of the FIROB ensures that noneof the write-back destinations are the same. The destination addressincludes the selected bytes for writing back data. In the case LIL andNC status bits are set for destinations of different size, and the FIROBmasks the selected bytes before sending to the register file. In somespecial case, the FIROB retires a partial line. The LIL and NC statusbits must be checked and reset before retiring, an extra cycle is neededfor this case. The LIL and NC status bits are also used for the flags tosimplify updating the flags registers.

A branch mis-prediction causes a partial line to be retired. Theregister file destination addresses are compared to all the previousdestinations within the partial line and reset the LIL and NC bitsbefore retiring the store instruction in the next cycle. The write-backdata is in the correct byte position with byte enables for the registerfile. The bytes are put in the correct position from the functionalunits, no shifting is needed in the FIROB.

Two PC's are kept for each line. The first PC is for the firstinstruction in the line, the second PC is for a second instruction whichcrosses the 16-byte boundary or the branch target within the line. ThePC offset is concatenated with line PC to get the current PC for retiredinstructions. A real register for the flags and program states isupdated with the retired instruction. As the taken branch instruction isretiring, the PC is updated with the branch target or next lineinstruction.

Updating the EIP Register

The EIP is used to keep track of the X86 instruction execution.Instructions can be aligned on any byte boundary so 32 bits of EIP areimplemented. Each retiring instruction updates the EIP register, andthere is no concept of freezing. The MROM instructions do not update theEIP register, indicatds by ROBEXIT, except for the last MROM instructionin the sequence. The EIP update falls into one of three categories:

Retire the entire line, no mis-prediction, EIP=Next line PC1.

Retire the line with branch mis-prediction, EIP=branch target.

Retire the partial line, ROBNPC=0, EIP=EIP: next entry PC offset.

Retire the partial line, ROBNPC=1, EIP=Next line PC2.

Floating-Point Interface

The FIROB keeps the floating point instruction pointer, data pointer,floating point status and opcode registers for floating pointinterfacing. The data pointer is updated on each floating point loadwith the instruction pointer and opcode registers are updated onfloating point exception. The data pointer is implemented in theload/store section. The floating point status register is implementedwith working and backup copies to ensure correct operation of floatingpoint stores.

Trap and Interrupt Processing

For internal exceptions from the functional units, LSSEC, and SRB, theexception entry in the FIROB will be retired in order. Similar to thebranch mis-prediction, the pipe and fetching should stop on an exceptionindication. When all entries before the exception entry have completedand retired, the exception procedure is initiated. All entries in theFIROB, the functional units, and LSSEC will be purged. The exceptionroutine will be fetched. The FIROB is responsible to generate the entrypoint to the MROM exception routine or new PC to the Icache. No state isupdated when a trap is taken. The processor fetches from an appropriateentry point and allows the microcode to perform the necessary statemodifications. It is up to the microcode to save the current EIP on thestack before the user's trap handler is called.

Dependency Checking & Dispatching

The FIROB is responsible for checking dependency of the source operandsof the dispatched line of instructions against the destination operandsof the previous lines of instructions. The dependency comparators areneeded for the first 5 lines in the FIROB and within the current line.The source addresses are checked for dependency against the previousdestination addresses, and the dependency tags are generated by the endof this cycle. The forwarding data from the FIROB is driven in the nextclock to the reservation station. Since the X86 instruction can onlyhave 2 operands, the operand can be both destination and source which isindicated by the operand's tag bits, the current destination addressesare checked for dependency against the previous destination addresseswith no extra cost in hardware. The write-after-write dependencyinformation is used to set three new status bits for most up-to-datedestination and the previously match destination. The three new statusbits are referred to as Last-In-Line (LIL) bit for each line, andNo-Compare (NC) bit for the whole FIROB. The NC bits ensure that thereis only a single HIT for any read operand. The NC bits are used toqualify the dependency checking comparator, and the LIL bits are used torestore the most up-to-date status of the destination in case ofpartial-line retiring by branch mis-prediction. Other ROBs can havemultiple HITs and search for the most current HIT. The procedure fordependency checking on this type of ROB is:

Compare source addresses to previous destination addresses.

Search for the most current HIT from multiple HITs.

Encode for the dependency tag.

Route tag and data to reservation station.

On Processor 500, the two intermediate steps are eliminated, since thereis only a single HIT, the HIT signal can be used as control to mux thetag and data directly. The NC and LIL bits are also used for retiringthe line to the register file. No two entries have the same destinationpointer for the line of instruction. The same method can be applied tothe status flags. The FNC and FLIL bits are used for the status flags.The status flags are in three groups, OF, {SF,ZF,AF,PF}, and CF.

As illustrated in FIG. 35, the matrix for dependency checking is 206-bit destination addresses against the 8 source operand addresses. Forread operand, if there is a hit in the FIROB, the FIROB has the highestpriority to drive the data on the operand bus to the functional unit.

If there is a branch mis-prediction, all instructions in the same lineafter the mis-predicted branch with the LIL bit set feed back to theRDOPTR to check and reset the previous entries with the NC bit set. Anextra cycle is used to retire instructions. The CANCEL status bits forall instruction after the branch mis-prediction should be used toqualify the instructions as NC.

Operand's Size Dependency

The size of the operands in dependency checking is quite complex for theX86 instruction set. Processor 500 checks for dependency from wide tonarrow and stalls the dispatch line in decode for the narrow to widedependency. Only one dependency tag is needed for each source operand.

Signal List

TOPPTR(2:0)--Pointer to the top of the FIROB. This pointer is used toenable the number of lines in the FIROB for dependency checking.

ENINTR(3:0)--Input from Idecode indicates external interrupt enable foreach instruction. This information is used for retiring instruction.

IDSIB(3:0)--Input from Idecode indicates which decode unit has theSIB-byte instruction. The FIROB should generate dependency on theprevious instruction of the last line in the same issue position.

MROMDEC(3:0)--Input from MROM indicates the microcodes are being decodedby the decode units. Use to set the ROBEXIT bit.

INSDISP(3:0)--Input from Idecode indicates that the instruction indecode unit is valid, if invalid, NOOP is passed to FIROB.

INSLSxB(5:0)--Input from decode units indicates the prefix values. bit5--data size, bit 4--address size, bit 3--lock, bit 2:0--segmentregisters.

IDSIB(3:0)--Input from Idecode indicates which decode unit has theSIB-byte instruction.

RBxTAG1(4:0)--Output indicates the first dependency tag for dispatchinginstructions.

RBxTAG2(4:0)--Output indicates the second dependency tag for dispatchinginstructions.

FCFxTAG(4:0)--Output indicates the CF flag dependency tag fordispatching instructions.

FOFxTAG(4:0)--Output indicates the CF flag dependency tag fordispatching instructions.

FXFxTAG(4:0)--Output indicates the CF flag dependency tag fordispatching instructions.

DSETALL(4:0)

DSETEXIT(4:0)

DSETINTR(4:0)--Input to set signals for dispatched instructions. Thebits should be set in the cycle after the dependency checking.

Handling of Load/Store

Handling of store and load-miss can also be done with the broadcastedentry. The FIROB broadcasts the next-in-line entry which does not yethave a the result from a functional unit. With this FIROB entry,WRPTR(4:0), the LSSEC knows when to execute the store or load-missinstruction without any handshake from the FIROB. This implementationwill eliminate the signals between the FIROB and the load/store unit. Anextra signal, CANENTRY, will accompany the WRPTR to indicate that thisentry has been canceled by mis-prediction. The LSSEC and the reservationstations can use this information to return the results without anyexecution, and any data dependency in the load/store buffer is ignored.The FIROB treats the load/store instructions the same as other ALUinstructions with this method.

The LSSEC implements a store buffer to keep track of the storeinstructions in the pipe for address dependency checking. The storebuffer is capable of issuing 2 store instructions to the data cache ifthe store instructions are next-in-line for retiring. To accomplishthis, the FIROB will broadcast another pointer, WRPTR1(4:0), andCANENTRY1 to the store buffer, if the first two entries in the storebuffer match the WRPTR and WRPTR1, two stores can be executedconcurrently as long as they accesses two different banks in the datacache. The WRPTR and WRPTR1 point to two entries in the FIROB which needresults. The two entries do not necessarily point to store instructions.

A similar procedure is also used for in-order execution of themove-to-special-register instructions. A buffer in the load/storespecial register is used for the special register instructions, the ROBtag is compare to the WRPTR to execute the instruction and return resultto the FIROB.

Load and store instructions assume single cycle access to the datacache. The results are speculatively latched into the data buffer. Thevalidation of the load/store is from the HIT signal of the data cacheearly in the next cycle. The result valid bits of the load/store isclear in the next cycle if miss in the data cache.

Unaligned Accesses

Processor 500 is optimized for aligned 8/16/32 bits accesses. Foraligned access, the operand comparison is for bit 31:2 with indicationfor checking of 8/16/32 bits. The LSSEC takes two or more clock cyclesto execute the instruction. The operation of the LSSEC for unalignedload is as followed:

First cycle: Access the data cache (DC) to read data with the currentaddress, another access may be concurrent. Increase the address by 4 forthe second access.

Second cycle: Access the DC to read data with the increased address,other access can be concurrent. Latch the data of the first access atthe beginning of this cycle. The DC puts the 8 or 16 bits at the leastsignificant byte (same as any narrow access), and the LSSEC puts thefirst half data of the unaligned at the most significant byte of theresult bus.

A miss in either part of the unaligned access must wait for the WRPTR orWRPTR1 of the FIROB to execute the instruction in program order. Theload/store buffer must keep the status to access the data cache and sendthe results correctly on the buses.

Both halves of the unaligned store are written into the cache at onetime to avoid any intermediate exception. The operation of the LSSEC forunaligned store is as follows:

First cycle: Access the data cache (DC) with the current address, do notwrite data, another access may be concurrent. If miss in the data cache,the reload routine may start this cycle. Increase the address by 4 forthe second access.

Second cycle: Access the DC using two ports to write data with bothaddresses

The advantages of the above procedure are:

The LSSEC always does two accesses and the unaligned access always takestwo cycles. The LSSEC has a whole cycle to increase the address; nodifferent for crossing the line boundary.

The LSSEC does all the merging for unaligned load with ease. The DC doesnot need to know about the unaligned access.

Signal List

LSTAG0(4:0)--Input from LSSEC indicates FIROB line number of the firstaccess.

LSTAG1(4:0)--Input from LSSEC indicates FIROB line number of the secondaccess.

LSRES0(31:0)--Input from LSSEC indicates result data of the firstaccess.

LSRES1(31:0)--Input from LSSEC indicates result data of the secondaccess.

WRPTR(4:0)--Output to LSSEC indicates the bottom (oldest) entry in theFIROB without valid result. If this entry matches the store or load-missentry in the LSSEC, the entry can access the data cache at this time.

CANENTRY--Output to LSSEC indicates the bottom entry in the FIROBwithout valid result is canceled. If this entry matches the store orload-miss entry in the LSSEC, the entry can return without access thedata cache at this time.

WRPTR1(4:0)--Output to LSSEC indicates the next to bottom entry in theFIROB without valid result. If this entry matches the store or load-missentry in the LSSEC, the entry can access the data cache.

CANENTRY--Output to LSSEC indicates the next to bottom entry in theFIROB without valid result is canceled. If this entry matches the storeor load-miss entry in the LSSEC, the entry can return without access thedata cache.

Handling of Branch and Re-synchronization

Since branches may be executed in parallel in multiple functional units,branch mis-prediction is handled in order. The mis-prediction ofbranches is handled by the FIROB. There are two types of branches, theconditional branch and the unconditional branch. The unconditionalbranch is always taken and includes call, return, and unconditionaljump. The mis-prediction is when the targets do not match. Forconditional branch, the mis-prediction is from taken/non-takenprediction and/or matching target address. Branch mis-prediction stopsthe pipe which can be done from the functional units, and allinstructions in decode are cleared, and the new target is fetched by theIcache at a later time. The functional units may send the mis-predictedsignal to stop the pipe and return the correct logical PC address to theFIROB. Because of the branch holding register, the branch mis-predictiondoes not update the ICNXTBLK until the next branch mis-prediction. TheFIROB can speculatively send the branch mis-prediction to the Icache. Ifthere is another mis-predicted branch prior to the last one, the FIROBcan invalidate the branch holding register in the ICNXTBLK. The Icacheuses an extra cycle to update the previously mis-predicted branch intothe ICNXTBLK, and the FIROB uses this cycle to update the NC status bitand retire the instruction to the register file. The new logical PCaddress from branch mis-prediction should be sent to the Icache andallow the code segment to be added to generate the linear address foraccessing the Icache. When an entry in the FIROB is completed withmis-prediction status, entries after the branch are marked withcancelled status. The cancelled entries can have incorrect result datafrom the functional units or LSSEC.

Another condition to re-fetch the instructions is re-synchronization.There is a possibility of executing a wrong instruction fromself-modifying code and updating the code segment register. The streamof instructions must be re-fetched. From external snooping which causean invalidation of a line in the instruction cache or an internal storeinstruction which can change a line in the instruction cache, the FIROBis re-synchronized at this point. As soon as the write to code-segmentregister is detected, following instructions are re-fetched uponcompletion of the code segment register write.

Recover of Status Bits from Branch Mis-prediction

The status bits are no longer correct with branch mis-prediction. TheFIROB takes one clock cycle to reset the status bits for each line ofthe FIROB after the branch mis-prediction. First, the line with thebranch mis-prediction will be corrected. Within the line if there is anyLIL bit is set after the branch mis-prediction, the destination iscompared against the previous destinations before the branchmis-prediction. A match will set the LIL bit and clear the NC bit forthat entry

Updating PC

Each line of the FIROB has two PCs, and each entry has a PC offset ofthe next instruction and one status bit to indicate if the offset isrelated to the second PC. As the line is retired, the current PC pointerwill point to the next line PC. For partial retiring of the line, theline PC is updated with the offset of the present entry in the FIROB.For the MROM entry, the offset is with the last MROM instruction, andall other MROM instruction should have the same offset with the line PC.With this technique, handling of the PC is relatively simple. In thecase of branch mis-prediction for sequential fetch, (the branchprediction is taken) the PC can be calculated by concatenating the linePC with the offset. If the PC is at the end of the line, the sequentialPC is +16.

Signal List

BRNMISP--Input from the Branch execution of the FU indicates that abranch mis-prediction. The Idecode clears all instructions in progress.

UPDFPC--Output to Icache indicate that a new Fetch PC has been detected.This signal accompanies the FPC for the Icache to begin access the cachearrays.

TARGET(31:0)--Output to Icache as the new PC for branch correction path.

BRNMISP--Input to Icache indicates a branch mis-prediction. The Icachechanges its state machine to access a new PC and clears all pendinginstructions.

BRNTAKEN--Output to Icache indicates the status of the mis-prediction.This signal must be gated with UPDFPC.

BRNFIRST--Output to Icache indicates the first or second target in theICNXTBLK for updating the branch prediction.

BRNCOL(3:0)--Output to Icache indicates the instruction byte forupdating the branch prediction in the ICNXTBLK.

FPCTYP--Input to Icache indicates the type of address that is beingpassed to the Icache.

BPC(11:0)--Output indicates the PC index and byte-pointer of the branchinstruction which has been mis-predicted for updating the ICNXTBLK.

Handling Traps and Interrupts

A function of the FIROB is to detect and prioritize the traps andinterrupts and to initiate specific redirections at appropriate times.The LSSEC and functional units send the highest exceptions to the FIROB.The basic mechanism for redirection is:

Assert EXCEPTION to clear out instructions in the pipe.

One cycle later, assert REQTRAP and drive new entry point to the MROM.

Correct look-ahead registers at decode and in the branch units.

The FIROB initiates the microcode routine from the MROM by REQTRAP anddoes not wait for LSSEC to be idle. There are three groups of traps andinterrupts:

Exception results from functional units, FPU, and LSSEC.

External interrupts (maskable and non-maskable).

Single step traps.

The FIROB includes a set of entry point vectors which can be sent toMROM on REQTRAP indication.

Internal Traps and Interrupts

The internal exception results are coded into 3 bits of ROBEXC:

000--no exception

001--load/store breakpoint

This is set when any load or store breakpoint status is returned. Theinstruction is retired normally. The debug entry point is generated andthe B bits of the debug status register are set according to the 2-bitdebug register hit code reported with the result. The redirection startswhen the whole instruction is completed; the ROBEXIT bit is set. Anothertrap or interrupt can have higher priority while the load/storebreakpoint is waiting for the rest of the instruction to complete. Thefloating point exception causes the FIROB to update all the floatingpoint exception registers but the debug trap has higher priority.

010--software interrupt with vector

This is set when a software interrupt status is returned.

This exception includes the INTO instruction. When the instruction isretired, the PC is updated and the exception with vector is taken.

011--floating point exception with write-back/push/pop This is set whenthe corresponding status is returned. The instruction retires normallywith the floating point opcode and instruction pointer registers areupdated. The FIROB does one of the four actions:

if a pending breakpoint exits, take a breakpoint trap.

if NE=1, take a floating point trap directly.

if NE=0 and IGNNE=0, freeze and wait for an external interrupt.

if NE=0 and IGNNE=1, resync to the next instruction.

100--exception with vector

This is set when an exception result is returned with a vector(including a TLB miss). When the instruction is retired, no write-backnor PC update occurs and the redirection with the entry point isinitiated. This is used for majority of traps, the entry point isprovided with the results.

101--exchange abort

This is set when an exchange abort status is returned. The retireprocedure is the same as exception with vector except that the PC isdriven back instead of the MROM entry point. The signal SYNC andEXCHGSYNC are asserted along with REQTRAP to notify the Icache.

110--not used.

111--floating point exception without write-back/push/pop

This is set when the corresponding status or an exchange result withunderflow is returned. The retire procedure is the same as the abovefloating point exception without write-back or push/pop.

External Interrupts

The external interrupts include both maskable and non-maskableinterrupts. The non-maskable interrupt (NMI) is a normal, precise,external interrupt. The NMI should only be seen by the FIROB. Theexternal interrupt is only recognized during selected windows:

Partially retired valid instructions in the bottom line with ROBEXITstatus and did not cause a trap or resynchronization.

Frozen due to having retired a floating point exception with NE=0 andIGNNE=0.

On external interrupts, the entry point is generated locally by theFIROB at the time the redirection is initiated. The maskable interruptis level sensitive while the NMI is edge sensitive. FLUSH and INIT arealso treated as edge sensitive asynchronous interrupts, similar to NMI.If the NMI is taken, it cannot be taken again before an IRET isexecuted. The microcode maintains a series of global flags that areinspected and modified by many of the trap handler entry points, and theIRET instruction. It is also the responsibility of the microcode todetect the NMI and delay the NMI until after executing of the IRET. TheMROM allows only one level of NMI. Many other aspects of nested trapcontrol (double fault, shutdown, etc.) will be handled with thismicrocode mechanism. There is no hardware support for this. When anenabled trap condition arises, the FIROB takes it at the next availablewindow.

The HALT instruction causes the FIROB to update the EIP before enteringshutdown mode. If the shutdown is entered as a result of a failed NMI,microcode should also clear the IF bit before halting.

Single Step Traps

When the TF bit of the EFLAGS register is set, a debug trap is taken atthe successful completion of each instruction, not including theinstruction that actually caused TF to be set (i.e. the POP or IRET).The FIROB takes a single step trap on the successful retirement of thesecond instruction after the setting of the TF bit. When the TF bit isclear the effect is immediate. When a single step trap is taken, theentry point is generated locally by the FIROB, and the BS bit of thedebug status register is set. The TF bit of the EFLAGS register is notcleared by hardware; it is cleared by microcode after pushing EFLAGSonto the stack.

Debug Interface

A summary of each type of debug trap is presented in this section. Loadand store breakpoints are detected by the LSSEC and returned as a statusto the FIROB with a 2-bit code identifying the breakpoint registermatched. When the instruction is retired the FIROB initiates a debugtrap and sets the corresponding B bit in the debug status register. Theentry point for this trap is generated locally. The instructions withload/store breakpoint trap are considered to have completedsuccessfully.

Instruction breakpoints are not handled by the FIROB. The pre-decodedisables the Icache and sends a special serializing instruction to eachnew instruction, whenever any of the debug registers are enabled forcode breakpoints. The special instruction is serially dispatched to theLSSEC where it compares the pre-decode PC to the breakpoint registers,accounting for the state of the RF bit. If a breakpoint is detected, anormal exception status is returned to the FIROB and a trap is taken.The provided entry point depends upon which breakpoint register got thehit, and the setting of the appropriate B bit is the responsibility ofthe microcode. The FIROB is unaware of the nature of the trap beingtaken. The RF bit is cleared automatically by the FIROB on thesuccessful retire of the second instruction following its low to hightransition.

Single step debug traps are handled in hardware by the FIROB as wasdescribed in previous section. The setting of the BS bit is doneautomatically by the FIROB but the handling of the TF bit is theresponsibility of the microcode.

The global detect debug trap is handled by the SRB, by inspecting thestate of the GD bit in the debug control register whenever a move to orfrom any debug register is attempted. If it is set, no move is performedand a trap status is returned. The setting of the BD bit when the trapis taken is performed by microcode; the FIROB is unaware of the natureof the trap being taken.

The task-switch debug trap is handled by microcode, including thesetting of the BT bit in the debug status register.

The breakpoint instruction (INT 3--0xCC) is treated like a normalsoftware interrupt. It is dispatched to a functional unit and returns anappropriate status. The FIROB updates the EIP register (which is onebyte for the INT 3 instruction) and traps to the provided entry point.The FIROB does not treat this instruction any different than othersoftware interrupts.

Signal List

NMI₋₋ P--Input from BIU indicates non-maskable interrupt, the FIROBgenerates a clean instruction boundary trap to a fixed entry point. TheFIROB is sensitive only to the rising edge of this signal

INTR₋₋ P--Input from BIU indicates the external interrupt. This signalis qualified with the IF bit of the EFLAGS register. The interruptoccurs at appropriate instruction boundaries.

SRBHALT--Input from SRB to enter HALT mode. The FIROB stops retiringinstructions until RESET, NMI, or external interrupt occurs. The FIROBmust retire the HALT instruction before shutting down.

CR0NE--Input from SRB indicates the NE bit of the CR0 register. The NEbit indicates the floating point exception can be trapped directly(NE=1) or via XFERR₋₋ P and an external interrupt (NE=0).

XIGNNE₋₋ P--Input from BIU indicates the copy of pin IGNNE. WhenCRONE=0, this signal is inspected to response to enabled floating pointexceptions.

XFLUSH₋₋ P--Input from BIU indicates an external flush request occurs.It is falling edge sensitive and trap on instruction boundary. It issample during IRESET to enter tri-state test mode, the FIROB should notgenerate exception.

IINIT--Input from BIU indicates an initialization request. It is risingedge sensitive and trap on instruction boundary. It is sample duringIRESET to enter BIST test mode, the FIROB generates on of the two resetentry point.

EFLAGSRF--Output generates from the EFLAGS register, these bits arevisible from the slave copy of the EFLAGS register. The RF bit is alsoused in the FIROB to handle instruction breakpoint.

EFLAGSIF--Output generates from the EFLAGS register, this is the maskbit for INTR₋₋ P. When clear, INTR₋₋ P is ignored.

EFLAGSTF--Output generates from the EFLAGS register, the interrupt andtrace flags are needed locally to control external interrupts and singlestep trapping after two completed instructions retires.

LOCVEC--Input from ROBCTL indicates whether entry point of theredirection is from the result status or locally generated.

ASYNCOK--Input from ROBWB indicates an external interrupt or NMI can betaken.

DOEXC--Input from ROBWB indicates an EXCEPTION is asserted and a trap tothe entry point returned with the instruction is initiated.

DOXABORT--Input from ROBWB indicates an EXCEPTION is asserted and aresync is initiated. The signal EXCHGSYNC is asserted in addition to thenormal resync signals.

DOFP--Input from ROBWB indicates an floating point exception byinspecting CRONE and XIGNNE₋₋ P. Exception, freeze mode, or resync istaken in next cycle.

DOBREAK--Input from ROBWB indicates an EXCEPTION is asserted and a trapto a locally generated debug entry point is initiated.

DOSBZ--Input from ROBWB indicates an EXCEPTION is asserted and a resyncto the next instruction is initiated.

DOLSYNC--Input from ROBWB indicates an EXCEPTION is asserted and aresync to the next instruction is initiated.

DOTRACE--Input from ROBWB indicates an EXCEPTION is asserted and a trapto a locally generated single-step entry point is initiated.

LOCENTRY(9:0)--Output of local entry point vector for traps orinterrupts.

EXCEPTION--Global output indicates that an interrupt or trap is beingtaken including resynchronization. Effect on Idecode and Fus is to clearall instructions in progress.

REQTRAP--Global output, one cycle after EXCEPTION, indicates that thetrap is initiated with new entry point or new PC is driven.

SYNC--Output indicates whether the new entry point or new PC is driven.

FREEZE--Output from a latch indicates when an SRBHALT occurs, or whenDOFP is asserted with CRONE=0 and XIGNNE₋₋ P=1. The latch is reset whenan enabled external interrupt, NMI, or IRESET occurs.

XFERR₋₋ P--Output to BIU indicates the floating point error which isinverted of the ES bit from the slave of the floating point statusregister. It is also used by the FIROB to generate the plunger traps.

EXCHGSYNC--Output indicates exchange instruction resynchronization toIcache. This occurs when an exchange with a masked underflow is retired.It is a special resynchronize exchange with alternate entry point.

Listing of Status Bits

This block describes the status bits and fields in the FIROB. The FIROBkeeps track of the processor states, status flags, handling correct PC,and retires instructions in program order to the register file. Thenumber of status bits and fields in the FIROB is organized in fourgroups: the data path, the entry's status bits, the line's status, andthe global field.

The Data Path

The data path contains the necessary data for the 32-bit communicationwith the decode units, the register file, and the functional units.

ROBDATA--RESULT DATA--32-bit--Receive data from functional unit bycomparison of result line number. Write data back to the register filefrom the bottom of the FIROB. This can also be the branch target to berouted to the Icache.

ROBDEST--DESTINATION OPERAND--6-bit--Receive the register address fromthe decode units into the top of the FIROB. Send the address to theregister file to latch valid result data from the bottom of the FIROB.The address routes to the comparators for dependency checking.

ROBTYPE--OPERAND TYPE--1-bit--Receive the type of the destinationoperand from the decode units. Indicates if ROBDEST is valid.

ROBBYTE--SELECT BYTE--3-bit--Receive the operand size from the decodeunits. Decode into 3 bits and for comparators and write back to theregister.

ROBNC--NO-COMPARE--3-bit--Received from the dispatch line comparator;indicates that there is another instruction in the dispatch line withthe same destination. The 3 bits is used for the bytes in the data word.This entry should not used in dispatch dependency checking. If theROBLIL bit is not set, the destination of this entry should not be usedfor writing back to the register file.

ROBLIB--LAST-IN-BUFFER BIT--3-bit--From comparing of the dispatch lineagainst the previous line in the FIROB. The 3 bits are used for thebytes in the data word. This entry ensures a single hit for lines in theFIROB. Use for dispatch dependency checking.

ROBLIL--LAST-IN-LINE BIT--3-bit--Received from the dispatch linecomparator; indicates that there is another instruction in the dispatchline with the same destination. The 3 bits are used for the bytes in thedata word. Use for writing back to the register file and for dependencychecking. If the ROBNC bit is also set; indicates the matcheddestination with another entry in the buffer.

ROBFNC--FLAG NO-COMPARE--5-bit--Received from the dispatch linecomparator, indicates that there is another instruction in the dispatchline with the same flag destination. This entry is not used in flagdependency checking. If the ROBFLIL bit is not set, the destination ofthis entry should not be used for updating the flags registers.

ROBFLIB--FLAG LAST-IN-BUFFER BIT--5-bit--From comparing of the dispatchline against the previous line in the FIROB. This entry to ensure asingle hit for all lines in the FIROB. Use for flag dependency checking.

ROBFLIL--FLAG LAST-IN-LINE BIT--5-bit--Received from the dispatch linecomparator; indicates that there is another instruction in the dispatchline with the same flag destination. Use for updating the flagsregisters and for dependency checking. If the ROBFNC bit is also set,indicates the matched destination with another entry in the buffer.

ROBPCOFF--PC OFFSET--4-bit--Received from the decode units, indicatesthe offset from the current line PC. This PC offset concatenates withthe PC to form the 32-bit address.

ROBTAG--FIROB TAG--3-bit--The hard-wired tag of the FIROB entries. Asingle tag is used for all lines in the FIROB. This tag in combinationwith the ROBLTAG is multiplexed to the reservation station in case ofdependency.

ROBFUPD--FLAG UPDATE--3-bit--Received from the decode units, indicatesthat the instructions will update the status flags. Use for flagdependency checking and writing back to the global status flagregisters. Bit 2--OF, bit 1--SF, ZF, AF,PF, bit 0--CF.

ROBFLDAT--FLAG RESULT--6-bit--Received from the functional units for theupdates flags. Use for writing back to the global status flag registers.

Signal List

RBOPOHIT1(3:0)--Input from ROBCMP indicates that the FIROB line 0matches with the first operand of the instruction at position 0. Thereare a total of 24 RBxPxHIT1(3:0) signals. These signals are used to muxthe dependency tag and data to the functional units.

RBlP0HIT1(3:0)

RB2P0HIT1(3:0)

RB3P0HIT1(3:0)--Input from ROBCMP indicates that the FIROB line 1-3matches with the first operand of the instruction at position 0.

RB0P1HIT1(3:0)

RB0P2HIT1(3:0)

RB0P3HIT1(3:0)

RB0P4HIT1(3:0)

RB0P5HIT1(3:0)--Input from ROBCMP indicates that the FIROB line 0matches with the first operand of the instruction at position 1-5.

RB0POHIT2(3:0)--Input from ROBCMP indicates that the FIROB line 0matches with the second operand of the instruction at position 0. Thereare a total of 24 RBxPxHIT2(3:0) signals.

RB1P0HIT2(3:0)

RB2P0HIT2(3:0)

RB3P0HIT2(3:0)--Input from ROBCMP indicates that the FIROB line 1-3matches with the second operand of the instruction at position 0.

RB0P1HIT2(3:0)

RB0P2HIT2(3:0)

RB0P3HIT2(3:0)

RB0P4HIT2(3:0)

RB0P5HIT2(3:0)--Input from ROBCMP indicates that the FIROB line 0matches with the second operand of the instruction at position 1-5.

WBENB(3:0)--Input from ROBCTL indicates that writing back is enable fromthe bottom of the FIROB.

RESx(31:0)--Input from FU indicates result data.

DTAGx(2:0)--Input from FU indicates FIROB line number of the result.

RFLAGx(31:0)--Input from FU indicates result flags.

LSTAG0(4:0)--Input from LSSEC indicates FIROB line number of the firstaccess.

LSTAG1(4:0)--Input from LSSEC indicates FIROB line number of the secondaccess.

LSRES0(31:0)--Input from LSSEC indicates result data of the firstaccess.

LSRES1(31:0)--Input from LSSEC indicates result data of the secondaccess.

WBXNC--Output indicates the invalid write-back data to the register filefor retiring instructions.

WBxPTR(5:0)--Output indicates the write-back pointer to the registerfile for retiring instructions.

WBxD(31:0)--Output indicates the write-back data to the register filefor retiring instructions.

WBxBYTE(3:0)--Output indicates the write-back selected bytes to theregister file for retiring instructions.

RBxDAT1(31:0)--Output indicates the first source operand data fordispatching instructions.

RBxDAT2(31:0)--Output indicates the second source operand data fordispatching instructions.

FLGxDAT1(5:0)--Output indicates the status flags for dispatchinginstructions.

RBxTAG1(4:0)--Output indicates the first dependency tag for dispatchinginstructions.

RBxTAG2(4:0)--Output indicates the second dependency tag for dispatchinginstructions.

FCFxTAG(4:0)--Output indicates the CF flag dependency tag fordispatching instructions.

FOFxTAG(4:0)--Output indicates the CF flag dependency tag fordispatching instructions.

FXFxTAG(4:0)--Output indicates the CF flag dependency tag fordispatching instructions.

TARGET(31:0)--Output to Icache indicates the new PC for branchcorrection path and resynchronization. It is also used for specialregister updates in the FIROB.

RBxNC--Output to ROBCMP indicates the invalid entry for dependencychecking.

RBxLIL--Output to ROBCMP indicates the last-in-line entry for dependencychecking.

RBxFNC--Output to ROBCMP indicates the invalid entry for flag dependencychecking.

RBxFLIL--Output to ROBCMP indicates the last-in-line entry for flagdependency checking.

ICPOSx(3:0)--ICLK7 Input from Icache to decode units indicates the PC'sbyte position of the next instruction.

IDxDAT(1:0)--Input from Idecode indicates the data size information.01-byte, 10-half word, 11-word, 00-not use.

IDxADDR--Input from Idecode indicates the address size information. 1-32bit, 0-16 bit.

DXUSEFL(2:0)

DxWRFL(2:0)--Input from Idecode indicates the type of flag uses/writesfor this instruction of decode units:

xx1 CF-carry flag,

x1x OF-overflow flag,

1xx SF-sign, ZF-zero, PF-parity, and AF-auxiliary carry

INSDISP(3:0)--Input from Idecode indicates that the instruction indecode unit is valid, if invalid, NOOP is passed to FIROB.

DxUSE1(1:0)--Input from Idecode indicates the type of operand being senton operand 1 for decode units. Bit 1 indicates source operand, and bit 0indicates destination operand.

DxUSE2--Input from Idecode indicates indicates source operand.

RDxPTR1(5:0)--Input from Idecode indicates the register address foroperand 1 of the instructions.

RDxPTR2(5:0)--Input from Idecode indicates the register address foroperand 2 of the instructions.

INSLSxB(5:0)--Input from decode units indicates the prefix values. bit5--data size, bit 4--address size, bit 3--lock, bit 2:0--segmentregisters.

The Entry's Status

Each entry of the FIROB has many status's.

ROBVAL--VALID RESULT--1-bit--Set when functional units return validresults. The entry can be retired when this bit is set. A NOOP sets thisbit during dispatching.

ROBTKN--TAKEN BRANCH--1-bit--Set when functional units return validresults. Use to update the EIP with the taken branch target.

ROBJMP--BRANCH INFORMATION--3-bit--Receive from the decode units,indicates CALL/RETURN (bit 0) or unconditional jump instruction (bit 1)and valid branch instruction (bit 2). The branch resets the branch tagin the global shift register and the return stack.

ROBGBTAG--GLOBAL BRANCH TAG--4-bit--Receive from the decode units,indicates the global branch prediction tag. Use to recover the globalbranch prediction shift register, the counters, and the byte position ofthe mis-predicted branch instruction. This is to properly update theICNXTBLK.

ROBCAN--CANCELED ENTRY--1-bit--Set when branch mis-prediction isdetected or SC-read₋₋ after₋₋ DC-write dependency is detected fromload/store dependency checking. The entry is retired normally withoutupdating the EIP.

ROBLSYNC--LOAD/STORE RESYNC--1-bit--Set when functional units returnvalid results with resync status. The load/store hits in the Icache forself-modifying code. The next instruction should be re-fetched from theIcache.

ROBSBZ--SHIFT BY ZERO--1-bit--Set when functional units return validresults with SBZ status. The scheduled flags updates are canceled. Thisstatus is used to qualify the ROBFUPD.

ROBEXIT--LAST MROM INSTRUCTION--1-bit--Set for instructions except forMROM instructions and SIB-byte instructions. This status is used toupdate the EIP and flags registers when retiring instructions.

ROBNPC--NEXT PC--1-bit--Set for instructions which use the second PC inthe line--ROBLPC2. This status is used to update the EIP with ROBLPC2when retiring instructions.

ROBEXC--EXCEPTION STATUS--2-bit--Set when functional units return validresults with exception status. The exception code is:

000--no exception

001--load/store breakpoint

010--software interrupt with error

011--floating point exception with write-back/push/pop

100--exception with vector (including TLB miss)

101--exchange abort

110--reserved

111--floating point exception without write-back/push/pop

ROBFP--FLOATING POINT ENTRY--1-bit--Set for floating point instructionduring dispatching.

Signal List

DSTATx(3:0)--Input from FU indicates the status of the result data:

0000--no result

0001--valid result

0010--valid result, shift by zero

0011--exception with vector

0100--software interrupt with vector

0101--TLB miss with vector

0110--load/store breakpoint

0111--exchange result

1000--exchange with underflow

1001--exchange abort

1010--branch taken, mis-prediction

1011--branch not taken, mis-prediction

1100--reserved for FPU

1101--reserved for FPU

1110--reserved for FPU

1111--reserved for FPU

DSETALL(3:0)

DSETEXIT(3:0)

DSETINTR(3:0)--Input to set signals for dispatched instructions. Thebits should be set in the cycle after the dependency checking.

RSETTKN(3:0)

RSETVAL(3:0)

RSETEXC(3:0)

RSETSBZ (3:0)

RSETLSYNC(3:0)--Input to set signals for result instructions. The bitsare set in the same cycle as the results from functional units.

WBALL(3:0)

WBVAL(3:0)

WBCAN(3:0)

WBTKN(3:0)

WBSBZ(3:0)

WBEXC(23:0)

WBEXIT(3:0)

WBNPC(3:0)

WBLSYNC(3:0)

WBFP(3:0)--Output indicates the current status of the bottom line of theFIROB for retiring instructions.

The Line's Status

Each line in the FIROB has its own status and information fordispatching and retiring instructions. The FIROB is a set of FIFOregisters to avoid the dependency checking for the top line. The virtualline tag which is routed to the functional units is rotated with theshifting of the line.

ROBPCL--LINE PC 1--28-bit--Receive from the decode units. This is the PCfor the first instruction in the line. Use to update the current retiredPC, branch mis-prediction, or re-fetch from Icache.

ROBPC2--LINE PC 2--28-bit--Receive from the decode units. This is whenthe instruction cross the 16-byte line boundary in the Icache, andbranch target of the instruction within the 4 entries in the ROB line.Use to update the current retired PC, branch mis-prediction, or re-fetchfrom Icache.

ROBPCB1--PC1 BRANCH TARGETS--2-bit--Receive from the decode units. Thisis information of the two branch targets in the ICNXTBLK for the linePC 1. For each bit, reset is non-taken or not valid, set is taken branchtarget. This information is for selecting a branch target forreplacement in case of branch mis-prediction on the undetected branch infetching.

ROBPCB2--PC2 BRANCH TARGETS--2-bit--Receive from the decode units. Thisis information of the two branch targets in the ICNXTBLK for the line PC2. For each bit, reset is non-taken or not valid, set is taken branchtarget. This information is for selecting a branch target forreplacement in case of branch mis-prediction on the undetected branch infetching.

ROBLTAG--VIRTUAL LINE TAG--3-bit--Reset for each line from 0 to 4. Thesevirtual line tags are rotated with retiring instructions. The line writepointer points to the bottom of the FIROB and the line read pointerpoints to the next available line in the FIROB. The virtual line tagsare sent to functional units.

ROBALL--ALLOCATE--1-bit--Set during dispatching of a line ofinstructions. Clear on retiring instructions. This status qualifies allother status's.

Signal List

IDPC1(31:0)--Input from Idecode indicates the current line PC of thefirst instruction in the 4 dispatched instructions.

IDPC2(31:0)--Input from Idecode indicates the current line PC of asecond instruction which cross the 16-byte boundary or branch target inthe 4 dispatched instructions.

ROBPC(31:0)--Output indicates the current retire line PC.

ROBEMPTY--Output indicates the FIROB is empty.

ROBFULL--Output indicates the FIROB is full.

LINEPTR(2:0)--Output indicates the current line pointer in the FIROB forthe dispatch line of instructions.

TOPPTR(2:0)--Pointer to the top of the FIROB. This pointer is used toenable the number of lines in the FIROB for dependency checking.

The Global Status & Registers

The FIROB includes some of the processor special registers. They areused for instruction execution. These registers can be accessed usingmove to/from protocol of the SRB. The special registers located in theFIROB are:

ROBEIP--PROCESSOR PC--32-bit--The register is updated on-the-fly byretiring instructions. It tracks the real instruction execution,regardless of the current state of the processor i.e. there is noconcept of freezing the PC. The EIP can be accessed using the standardmove to/from protocol of the SRB.

RCVBASE--RECOVERY PC BASE--32-bit--Update at the retirement of eachtaken branch instruction by the content of the ROBEIP(31:4) and theoffset of the branch instruction within the line. It is used bymicrocode to recover the PC of a branch to an illegal address. This isnecessary since the limit violation is not detected until the branchinstruction is fetched.

EFLAGS--THE FLAG REGISTER--19-bit--Update at the retire of theinstructions. The six status bits are divided into three groups OF,{SF,ZF,AF,PF}, and CF. The RF bit is cleared during certain debugoperations. EFLAGS bits are cleared by IRESET. The non-status bits canbe accessed via the move to/from protocol of the SRB by 10 differentpointers. These ten pointers provide independent software read/writeaccess as:

read/write the entire EFLAG register--bits 18:0.

read/write the lower word--bit 15:0.

read/write the lower byte--bit 7:0.

complement the carry flag--bit 0.

set/clear the direction flag--bit 10.

set/clear the interrupt flag--bit 9.

set/clear the carry flag--bit 0.

FPIP--FLOATING POINT PC--48-bit--Update at the retirement of floatingpoint instructions. The FPIP can be accessed using the standard moveto/from protocol of the SRB.

FPSR--FLOATING POINT STATUS REGISTER--16-bit--Update at the retirementof floating point instructions. The FPSR can be accessed either by usingthe standard move to/from protocol of the SRB or by a unique pointer toclear the exception bits. A move to FPSR must be accompanied by a moveto the look-ahead copy which is the responsibility of the microcode.

FPOPCODE--FLOATING POINT OPCODE REGISTER--11-bit--Update at theretirement of floating point instructions. The FPOPCODE can be accessedusing the standard move to/from protocol of the SRB.

DR6--DEBUG STATUS REGISTER--16-bit--Update the B bits at the retirementof the load/store breakpoints instruction and update the BS bits duringsingle stepping. On instruction breakpoints, global debug traps, andtask switch debug traps, DR6 must be set by microcode. The DR6 can beaccessed using the standard move to/from protocol of the SRB.

Signal List

WRFPSR(1:0)--Input from ROBCTL indicates to write the two floating pointflag groups, {C3,C2,C1,C0} and {SF,PE,UE,OE,ZE,DE,IE}. The updating ofFPSR register is from FPSRIN.

FPSRIN(10:0)--Input data for FPSR register updates.

WRFPOPCD--Input from ROBCTL indicates to write the FPOPCODE registerfrom FPOPCDIN.

FPOPCDIN(10:0)--Input data for FPOPCODE register updates.

PUSHPOP(2:0)--Input to increment or decrement the TOP field of the FPSRregister. Bit 0--push, decrement by 1. Bit1--pop, increment by 1. Bit2--double pop, increment by 2.

WRxFLG(2:0)--Input from ROBCTL indicates to write the three flags ofEFLAGS register.

EFTOFLGB(2:0)--Input from ROBCMP indicates to drive the flags tofunctional units on flag dependency checking.

CLRRF--Input from ROBCTL indicates to clear the RF bit of EFLAGSregister.

UPDFPIP--Input from ROBCTL indicates to update FPIP from LSCSSEL andEIP.

SETBS--Input from ROBCTL indicates to update the B bit of DR6.

LSCSSEL(15:0)--Input from LSSEC indicates the current code segment usedfor updating FPIP.

WRPC(3:0)--Input from ROBCTL indicates which PC offset to use to updateEIP.

RBLPC(31:4)--Input from the next to bottom line PC for updating of EIP.

MVTEIP--Input ROBCTL indicates EIP register updates from IAD bus.

MVFEIP--Input ROBCTL indicates EIP register move to IAD bus.

MVTCVB--Input ROBCTL indicates RCVBASE register updates from IAD bus.

MVFCVB--Input ROBCTL indicates RCVBASE register move to IAD bus.

MVTCVIO--Input ROBCTL indicates RCVIO register updates from IAD bus.

MVFCVIO--Input ROBCTL indicates RCVIO register move to IAD bus.

MVTIPCS--Input ROBCTL indicates the upper 16 bits of the FPIP registerupdates from IAD bus.

MVFIPCS--Input ROBCTL indicates the upper 16 bits of the FPIP registermove to IAD bus.

MVTIPOFS--Input ROBCTL indicates the lower 32 bits of the FPIP registerupdates from IAD bus.

MVFIPOFS--Input ROBCTL indicates the lower 32 bits of the FPIP registermove to IAD bus.

MVTDR6--Input ROBCTL indicates DR6 register updates from IAD bus.

MVFDR6--Input ROBCTL indicates DR6 register move to IAD bus.

MVTEFLAGS(2:0)--Input ROBCTL indicates EFLAGS register updates in threepieces (the upper half-word and the lower two bytes) from IAD bus.

MVFEFLAGS(2:0)--Input ROBCTL indicates EFLAGS register moves in threepieces (the upper half-word and the lower two bytes) to IAD bus.

MVTEFBIT(6:0)--Input ROBCTL indicates manipulation of individual bits inthe EFLAGS register. The action performed for each of these bits is:

bit 6: complement the carry flag (bit 0)

bit 5: set the direction flag (bit 10)

bit 4: set the interrupt flag (bit 9)

bit 3: set the carry flag (bit 0)

bit 2: clear the direction flag (bit 10)

bit 1: clear the interrupt flag (bit 9)

bit 0: clear the carry flag (bit 0)

MVFDR6--Input ROBCTL indicates DR6 register move to IAD bus.

EFLAGSAC

EFLAGSVM

EFLAGSRF

EFIOPL(13:12)

EFLAGSOF

EFLAGSDF

EFLAGSAF

EFLAGSCF--Output generates from the EFLAGS register, these bits arevisible from the slave copy of the EFLAGS register. The RF bit is alsoused in the FIROB to handle instruction breakpoint.

EFLAGSIF

EFLAGSTF--Output generates from the EFLAGS register, the interrupt andtrace flags are needed locally to control external interrupts and singlestep trapping.

XRDFLGB(3:0)--Output to flag operand bus, the bits are read by EFTOFLGB.The order of the bits is OF, SF,ZF,AF,PF,CF.

MVTFPSR--Input ROBCTL indicates FPSR register updates from IAD bus.

MVFFPSR--Input ROBCTL indicates FPSR register move to IAD bus.

CLRFPEXC--Input ROBCTL indicates to clear the stack fault and exceptionbits {SF,PE,UE,OE,ZE,DE,IE} in the FPSR register. Indirectly the ES andB bits are cleared.

FPTOP(2:0)--Output to register file indicates the current top-of-stackto identify the registers being popped to clear the full bits.

REQTRAP--Global output, one cycle after EXCEPTION, indicates to drivethe XLASTKPTR.

XFERR₋₋ P--Output to BIU indicates the floating point error which isinverted of the ES bit from the slave of the FPSR. It is also used bythe FIROB to generate the plunger traps.

XLASTKPTR(2:0)--Output to Idecode indicates the TOP bits for the FPSRfor correct floating point stack pointer.

MVTFPOPCD--Input ROBCTL indicates FPOPCODE register updates from IADbus.

MVFFPOPCD--Input ROBCTL indicates FPOPCODE register move to IAD bus.

Timing

Since the clock cycle is short, the comparators and detection of the hitfor dependency can be done in a phase using CAM cells. The tag anddecision to send data should be done in another cycle.

ICLK2: The operand linear address and register address is available atthe end of this cycle.

ICLK3: Dependency checking. Generate dependency tag and read data tooperand steering if hit.

ICLK4: Read and mux data to the operand data buses to the functionalunits. Update status bits.

For retiring instructions:

ICLK5: Results from the functional units. Compare tag to latch data.Update status and check for branch mis-prediction.

ICLK6: Retire a line if all entries have valid results. Update PC,flags. Drive new WRPTR and WRPTR1.

Layout

The layout of the FIROB is in three locations:

The operand addresses and comparators in the data path next to thedecode units.

The result data in the data path next to the reservation stations andfunctional units.

The status and global controls on the left side of the data path.

A block diagram of the layout is shown as FIG. 36.

Register File Overview

Processor 500 has the standard x86 register file (EAX to ESP) which isread from four dispatch positions and written to from the FIROB usingfour writeback positions. It is the FIROB's responsibility to make surethat no two writebacks go to the same register. There are also 16scratch registers available to all 4 dispatch positions. Eight registersare currently in the design for microcode use. Only the real(non-speculative) states are stored in the register file. No floatingpoint registers are stored in the integer register file. Each of the 8visible registers and the 16 temporary registers have enables toselectively write/read to bits (31:16), (15:8), or (7:0). The FIROB sendbytes enable bits and valid write bits to the register file. Read validbits and read byte enables will be sent by the dispatch/decode unit.Constants are not handled in the register file but rather get sent onthe BOPND bus using the decode unit.

Registers:

Address Register Name

    ______________________________________                                        00                 EAX                                                        01                 EDX                                                        02                 ECX                                                        03                 EBX                                                        04                 EBP                                                        05                 ESI                                                        06                 EDI                                                        07                 ESP                                                        .                                                                             .                                                                             10                 TMPREG00                                                   11                 TMPREG01                                                   12                 TMPREG02                                                   13                 TMPREG03                                                   .                                                                             .                                                                             .                                                                             1F                 TMPREG0F                                                   ______________________________________                                    

Register File Timing

A register file timing diagram is illustrated in FIG. 37. Instead of thenormal method of a write to the register file followed by a read fromthe register file, Processor 500 does a read first followed by a write.The early read allows sufficient time for multiplexing between theregister value (the one actually read) and forwarding from the incomingwrites. The end of the cycle is needed to drive the read value over tothe operand steering unit. A self timing circuit is used to providesufficient delay for the write and read decoding logic to completebefore the read and write actually take place. Both the read and writedecoding sections start decoding immediately after the ALAT's latch inthe read and write pointer busses. There are sixteen 5 bit comparatorswhich detect if forwarding is needed from a writeback port to a readport. The forwarding will bypass the delay through the register latchand help allow the read to complete within the cycle. The Read outputsfrom the register file will drive on a dedicated bus over to the operandsteering unit. For maintaining fast logic, 3 input nand gates are usedin the decode section. The 4 input nand gate is a large decrease inspeed, and the is 5 bit pointer bus along with an enable signal fit atwo 3 input nand gate structure. There is not any reset logic for theregister array.

Register File Sub Blocks

A block diagram of the register file sub blocks is given as FIG. 38. Theregister sub-blocks are the register cell array with four data inputsand eight data outputs, the compare array, the read decode array, thewrite array, the muxing section, and the output drivers. The ability toboth read and write in during the same cycle is important to this block,and careful attention needs to be paid to the layout, capacitive loadingand the bussing. Due to problems with fringing capacitance betweenadjacent metal lines, the register file does not containprecharge/discharge logic. It makes use of multiplexers, tristate gates,and static logic.

Signal List

RDnPTR1(4:0)--the first operand pointer for reading from the registerfile for positions 0 to 3.

RDnPTR2(4:0)--the second operand pointer for reading from the registerfile for positions 0 to 3.

USE1RD(3:0)--These signals are valid bits from IDECODE indicating whichreads are valid for the 1st operand. Each bit in these busses correspondto a dispatch position.

USE2RD(3:0)--These signals are valid bits from IDECODE indicating whichreads are valid for the 2nd operand. Each bit in these busses correspondto a dispatch position.

RDnENB1(2:0)--byte enables for position n and for the 1st operand. Bit 2refers to the upper two bytes while bits 1 and 0 refer to the lowerbytes (bits 15:8) and (bits 7:0).

RDnENB2(2:0)--byte enables for position n and for the 2nd operand. Bit 2refers to the upper two bytes while bits 1 and 0 refer to the lowerbytes (bits 15:8) and (bits 7:0).

WBnPTR(4:0)--the writeback pointer for position n. This must bequalified with the register write valid bits.

VRWB(3:0)--valid register writeback indication for each of sixpositions.

WBnENB1(2:0)--byte enables for position n and for the registerwriteback. Bit 2 refers to the upper two bytes while bits 1 and 0 referto the lower bytes (bits 15:8) and (bits 7:0).

RDnREGA(31:0)--read data from the register file for position n andoperand A.

RDnREGB(31:0)--read data from the register file for position n andoperand B.

WBnD(31:0)--writeback data from the FIORB for position n.

LOAD/STORE SECTION Overview

The load store section in Processor 500 can perform single-cycle accessof two memory based operands (2 loads or 1 load and 1 store). It canalso perform out-of-order loads requested by the functional units. Thestores go in-order and are performed as pure writes. The data cache is alinear cache, dual ported for the two concurrent accesses, 16 KB 8-wayset associative with way prediction. FIG. 39 shows the basicorganization of the load/store section. It is comprised of a specialunified load-store buffer. The information on whether an instruction isa load or a store is sent to the LSSEC by the decode unit. The linearaddress and data are computed by the functional units and sent to theLSSEC on the RESLA and result buses. The load/store unit then performstwo data cache accesses. The loads may be performed out-of-order, butthe stores are always in order. The unified load-store buffer is8-entries deep. It can accept up to four instructions per cycle.

Other Features are:

Unaligned accesses have at least one cycle penalty (2-cycle latency).

The current scheme supports non-blocking loads.

Note: Unaligned accesses in this section means accesses crossing doubleword boundaries.

Unified Load-Store Buffer (LDSTBUF)

The loads and stores are buffered up in a common queue, called theunified load-store buffer. The load and store instructions are bufferedup in this common queue in program order with the earliest instructionat the bottom and the latest instruction at the location pointed to byLDSTPTR-1, as shown in FIG. 40.

The advantages of this scheme over the conventional scheme where theloads and stores are buffered up in different queues are:

1. Efficient utilization of space due to which the dispatch would stallbased on the total number of loads and stores as opposed to the numberof loads or number of stores in the conventional method.

2. Also, since communication is between the decode unit and load-storesection, the functional units would never stall. They can keep executingthe instructions in their reservation stations.

3. The order of loads and stores are known since they are dispatcheddirectly to the load-store section, instead of going through thefunctional units which could send requests out of order.

Each entry (as shown in FIG. 41) in the buffer is broken down into threefields. The first field is made up of the FIROB instruction tag and theinstruction type (load, store or load-op-store). The source of updatesfor this field is the decode unit/FIROB. The second field has the linearaddress and store data and the associated valid bits, the update sourcebeing the functional units. The third field is made up of some controlinformation (for e.g. M bit indicating that this entry missed in thedata cache on a prior access, D bit indicating that the load in theentry is dependent on a store in the buffer), the update source beingthe load-store section itself.

LSCNT 2:0! indicates to the decode unit the number of free entries inthe buffer so that decode can take the necessary action. It is thedifference between 8 and LDSTPTR.

Updating the Entries

FIG. 42 illustrates a timing diagram of when the different fields ofeach entry in the buffer are updated. The instruction types (ITYPEnB1:0!) and ROB tags (DTAGnB 2:0!) are sent in a given(i)th ICLK. Thecorresponding mux select lines are generated and the entries updated.Also the LDSTPTR is incremented/decremented and by the end of that cycleLSCNT 2:0! is generated. At the beginning of the (i+1)th ICLK, thefunctional units send the tags of the instructions they are currentlyprocessing. The LSSEC looks at the tag information from the functionalunits, compares against the DTAG information in the LDSTBUF and sets upthe appropriate mux select lines to latch in the linear address andstore data. Also, the corresponding valid bits (LV and DV) are set up atthis point so that they can be latched when the address and data getlatched on the rising edge of (i+2)th ICLK. The load-store section thenaccesses the data-cache.

Prioritizing the Accesses

The accesses to be performed sit in the unified load-store buffer withor without the linear addresses and store data. The load-store sectionis able to forward the linear addresses from the functional units fordata cache accesses. In general, the processing of instructions isprioritized according to the following:

1. Misses and stores have the highest priority. They are processed assoon as they are known to be non-speculative. The reason that the storeshave higher priority over loads is because of the line-oriented natureof the reorder buffer and we would want to retire instructions asquickly as possible.

2. Loads can go out of order. The linear address for a load can be inthe buffer or can be forwarded directly from the functional units. Theresult can come back from the data cache or the unified load-storebuffer from stores that have not been retired. The results are returnedon the dedicated load-store result buses LSRES0 and LSRES1.

Forwarding the linear address from the functional units directly fordata cache accesses is based on the assumption that there is no validoutstanding load in the buffer. If there is any load (even one) in thebuffer, it is sent to access the data cache and the linear addressesfrom the functional units are latched in the LDSTBUF. This scheme isfollowed because of cycle time constraints.

Data-Cache Accesses

The data-cache accesses are performed through the LSLINAD0 and LSLINAD1buses. LSLINAD0 is connected to port A and LSLINAD1 is connected to portB of the data cache. The results are returned on LSRES0 and LSRES1buses. The data for store accesses is driven on SBADAT and SBBDAT buses.

Hits

Bits 13:0 of the linear address are presented to the data cache justbefore a given clock edge, so that the DCACHE has enough time to latchit. The array is accessed and the tag is compared with the upper 17-bitsof the linear address to generate the hit signal. The data is driven tothe functional units prior to the hit being generated. A cancel signalis sent in the next cycle based on whether the access hits/hits inunpredicted way/misses.

Misses

If there is a miss, the M bit (Miss) for that entry is set. Theload/store section based on the WRPTR information determines whether theinstruction is non-speculative in order to go external to fetch thedata. There is no handshaking between LSSEC and FIROB. Once the data iswritten to the data cache, the LSSEC performs the access again.

Handling Branch Mispredictions

Whenever a branch misprediction is encountered, the FIROB asserts theBRNMISP signal. The load-store section then sets a latch based on thissignal. When a new load or store instruction gets dispatched, theload-store section sets the NF bit for all stores in the buffer andclears the latch. Setting the NF bit for a store indicates that the datafrom that store should not be forwarded. If there is a load that isdependent on a store with the NF bit set, it is serialized by settingthe S bit. The loads keep returning results as they would normally. Thestores and load misses are canceled when the FIROB asserts CANENTRY forthat instruction.

Handling Misses

After a data cache miss, the load-store section performs speculativeMMU/PT accesses for possible alias. If an alias exists, the data cachetag is updated and the access performed again. If cache master reports amiss in the physical tags, the load-store section marks the M bit in theentry. The miss is processed after the instruction becomesnon-speculative. This is similar to stores based on the WRPTRinformation.

If the CMASTER reports that the line is in the ICACHE (self-modifyingcode) and if the access happens to be a store, the LSSEC sends a"store-with-resync" status to the ROB to flush out the instructionsafter the store.

Unaligned Accesses

Unaligned loads are performed as 2-cycle single port accesses. When aload access is unaligned, the LSSEC splits that access into 2 singleport accesses and issues them in two separate cycles to the data cache.Another aligned load or store can accompany the access on the otherport. In the first cycle as the first half of the access is occurring,the address is incremented by 4 and in the second cycle, the other halfof the access is performed. The merging is done by the LSSEC.

Unaligned stores as single port accesses have a problem if they cross aline boundary. If one access hits and the other misses, and if a faultoccurs while processing the miss, it could leave the processor in anundesirable state. The safest and probably the easiest approach is tolet the LSSEC perform a dual port access so that if any of the twoaccesses misses, the other is canceled and is performed only after theappropriate line is brought in.

Checking for Unaligned Accesses

To determine whether an access is unaligned or not, the LSSEC detectsthe least significant two bits of LSLINAD0 and LSLINAD1 and thecorresponding data operand sizes. An access is unaligned if the operandsize is 32-bits and any one of the two least significant address bits isset or if the operand size is 16 bits and both the least significantbits of the linear address are set.

Aliasing Problem and Solution

The data cache is a linear cache with two ports. There is a potentialthat two or more linear addresses map to the same physical address(aliasing). There can never be more than one location in the data cachefor a physical address. The problem is with the loads goingout-of-order. A load with linear address LA1 has the potential of goingout-of-order and if that location exists in the data cache, it hits andreturns the result. A store with linear address LA2 ahead of the loadmight be mapped to the same physical address. Then, the result that theload returned is wrong since the load was not able to see thedependency. The solution to the problem is the way load-store dependencychecking is performed (described later).

Non-blocking loads

The gap between microprocessor and memory speeds is widening with everynew generation of microprocessors. This speed discrepancy can impactperformance if the load-store section of the processor stalls whenever adata cache miss occurs. To prevent stalling of the load-store section,loads in the buffer can access the data cache as any miss is beingprocessed. The following describes, in brief, the method that Processor500 uses to perform non-blocking loads.

Load and store instructions in Processor 500 are sent to a unifiedload-store buffer. Each entry in this buffer has a bit M that indicatesif the load or store in the entry missed on a prior access. Whenever adata cache access misses, the M bit (miss) is set in the entry. Sincethe processor allows speculative execution, the miss cannot be processedimmediately. It can be processed only when the instruction is no longerspeculative. In conventional implementations, when a miss is beingprocessed, loads or stores in the queue are held up till the miss getsresolved. The load-store section requests the Cache Master (CMASTER) toperform an external access to process a miss. The CMASTER goes externaland takes a long time before it can fetch the line of interest. Thefetched line comes in packets of 64 bits over the data bus to CMASTERwhich in turn sends the packet over to the data cache. The data cachelatches the packets in a local buffer (BUF1). In the mean time, theload-store section can keep sending load accesses to the data cache. Ifthe accesses hit in the cache, the results are returned. If an accessmisses, the M bit for that entry is set and the remaining loads in thebuffer can be sent to the data cache. Once the data cache receives theentire line from CMASTER, it initiates the reload sequence. This reloadsequence involves reading the line to be replaced into another localbuffer (BUF2) and then updating the line and tag with the contents ofBUF1. When it performs the line write, the load-store section clears theM bit for all the entries in the LDSTBUF, so that if any of the accessesmissed on the line that was being brought in, it need not go externalagain. If the line in BUF2 is dirty, the load-store section initiates anexternal write cycle through the CMASTER.

Possible Data Cache Access Scenarios

Port A:

Reloads from Physical Tags (PT)

Accesses from LSLINAD0

Drives result on LSRES0

Port B:

Reload invalidations

Accesses from LSLINAD1

Drives result on LSRES1

Before accessing the data cache, the load-store section will check forbank conflicts and unaligned accesses. If there is a bank conflict, theload-store section will issue only one access. This is transparent tothe data cache.

Port A--load, Port B--load

Port A hit, port B hit--Complete both accesses

Port A miss, port B hit--Complete Port B access, mark entry as miss

Port A hit, port B miss--Complete Port A access, mark entry as miss

Port A miss, port B miss--Mark entries as misses

Port A access unaligned--Perform access. Take penalty.

Port B access unaligned--Perform access. Take penalty.

Port A--store, Port B--load

Port A hit, port B hit--Complete both accesses

Port A miss, port B hit--Complete Port B access, mark entry as miss

Port A hit, port B miss--Complete Port A access, mark entry as miss

Port A miss, port B miss--Serialize

Port B access unaligned--Perform access. Take penalty.

Port A--load, Port B--store

Same as the previous case.

Port A--store, Port B--store

The ROB provides two signals: WRPTR--the current instruction (store) tobe retired and WRPTR1--the next store in the current line that can beretired. By looking at this information, the LSSEC should be able toperform two simultaneous stores to the DCACHE.

Port A hit, port B hit--Complete accesses

Port A miss, Port B hit--Complete Port B access

Port A hit, port B miss--Complete Port A access

Port A miss, Port B miss--Serialize

Port B access unaligned--do not perform access

FIG. 43 gives the timing of aligned data cache accesses. As can be seen,there are three different cases--hit, hit in unpredicted way and miss.The data cache uses a way prediction scheme to access its data and tagarrays. If the prediction is wrong and the data happens to reside in thecache, a 1-cycle penalty is taken. The result is returned in the nextcycle. It is also important to cancel the previous result.

Unaligned accesses are more complicated. The following flow charts givethe details. Note that cycles for speculative MMU/PT accesses are nottaken into consideration in the flow charts.

Unaligned Load:

Single Port access

Cycle 1: Perform MA1 access, increment address

Cycle 2: Hit/Hit in unpred way/Miss indication for MA1

Perform MA2 access

Cycle 3: Hit/Hit in unpred way/Miss indication for MA2

If MA1₋₋ hit,

Perform next access

If MA1₋₋ hit₋₋ in₋₋ unpred₋₋ way,

Perform MA2 again

If MA1₋₋ miss,

Mark M bit

Cycle 4: If MA2₋₋ hit,

Instruction out of buffer

If MA2₋₋ hit₋₋ in₋₋ unpred₋₋ way,

Perform next access again

Instruction out of buffer

If MA2₋₋ miss,

Mark M2 bit.

Unaligned Store

2-port access

Cycle 1: Unmerge data, increment address

Cycle 2: Perform dual port access

Cycle 3: Hit/Hit in unpred way/Miss indication for MA1 and MA2. Performnext access

Cycle 4: If MA1₋₋ hit and MA2₋₋ hit,

Instruction out of buffer

If MA1₋₋ hit and MA2₋₋ hit₋₋ in₋₋ unpred₋₋ way,

Perform next B access again

Instruction out of buffer

If MA1₋₋ hit₋₋ in₋₋ unpred₋₋ way and MA2₋₋ hit,

Perform next A access again

Instruction out of buffer

If MA1₋₋ miss,

Mark M bit

If MA2₋₋ miss,

Mark M2 bit

Checking for Bank and Index Conflicts

For circuit reasons, two concurrent accesses to the data cache cannot beto the same bank. Therefore, the LSSEC must determine if there is a bankconflict and cancel the port B access and issue it in the next cycle.Since the tag array is also dual ported, the data cache needsinformation if two concurrent accesses are to the same index.

Bank Conflict: LSBNKCT is driven high if LSLINAD1 4:2!==LSLINAD0 4:2!

Index Conflict: LSINDXCT is driven high if LSLINAD1 10:5!==LSLINADO10:5!

Also for the way prediction array, which is also dual ported, anothersignal LSPRWYCT is driven if LSLINAD1 13:5!==LSLINAD0 13:5!.

Dependency Checking Against Stores in the Buffer

When the load-store section performs data cache accesses, it alsoperforms dependency checking against the stores in the store buffer.Bits 11:2 of the linear addresses are compared against the correspondingbits of the linear addresses of the stores ahead of the load Also, theoperand sizes and other misaligned address information are used in theequation to figure out dependency!. If the addresses match, the data ofthe latest store to that address is forwarded on to the result bus. Toperform dependency checking and forward the store data, the load-storesection has one whole cycle. In the following cycle, the rest of thebits (31:12) are compared. If this compare fails, the LSSEC signalsFIROB and the functional units to cancel that result just the way thedata cache does and the LSSEC serializes that load.

The advantage of using 10/12-bit comparators instead of 32-bitcomparators is the reduction in hardware and increase in speed. Bycomparing bits 11:2, and serializing loads that have partial addressmatches, the the aliasing problem may be solved.

On the condition that the addresses match and the operand sizes aredifferent, the load is serialized. A timing diagram of dependencychecking is given as FIG. 44. A hardware block diagram is given as FIG.45.

Summary of dependency checking/store forwarding:

1. Forward data operand from a store if:

the load and store addresses match completely, store data is valid andthe operand sizes are equal.

This is true if both the load and store accesses are misaligned.

2. Set `D` bit for the load if:

the store address is not computed yet

or

the addresses match, opsizes match and data is not valid.

The D bit is cleared on any store data or address update from thefunctional units.

3. Set `S` bit if:

the addresses match and opsizes are different

or

the load access is to the same bank as the store and not to the samebyte (partial dependency)

or

the load access is to the adjacent bank as a misaligned store (partialdependency--tricky case)

S -->this load is serialized--wait till load is at the bottom of theload-store buffer.

Snooping the Load-Store Buffer

During prefetch, the CMASTER snoops the LDSTBUF entries to check ifthere are valid pending stores to that line. If there is a pending loadto the line, then the prefetch holds till the store is done. To limithardware supporting this feature, eight dedicated 8-bit comparators areprovided for partial comparison of the line address. This structure canbe used for the general snooping issues.

No snooping structures are required in the load-store buffer. Thereasoning goes as follows:

In previous processors, a store sitting in the store buffer is retiredas far as the ROB and the execution core of the machine are concerned.The ROB can retire instructions that are past the store even before thestore is written to the data cache.

During prefetch of a cacheable line, the store buffer entries need to besnooped make sure that a store in the store buffer is not to the sameprefetched line. If it is (self-modifying code case), the prefetchstalls till the store is written.

The reservation stations need not be snooped in this case since thestore has not performed the data cache lookup yet. When the store doesthe lookup (a speculative read), it is going to miss in the data cacheand the Physical Tags (PT) will assert IC₋₋ CACHED signal indicating theLSSEC to return with the "store with resync" status. The ROB will thenflush all instructions after the store and instructions areredispatched.

If the prefetched line is non-cacheable, prefetch stalls if there arestores in the store buffer to the same line or if there is a store inthe reservation stations. The reason for making sure that thereservation stations do not have stores is that Physical Tags (PT) willnot assert IC₋₋ CACHED since the prefetched line is not cached and mightnot be in the prefetch buffer.

Serialization Conditions

a) Miss--Any entry that misses has to wait for WRPTR from FIROB tocompare against its tag before it can go external.

b) Store--Wait for WRPTR from FIROB.

c) Store address match, data not ready--Wait for store data.

d) Store address match, data ready but of different size--wait till thestore is done.

e) Store linear address not ready--wait for store linear address.

f) Store address match, NF bit set.

g) Bits 11:0 match and 31:12 don't--possible alias.

Handling Non-Zero Segment Bases

The discussion above assumes that the linear addresses are sent over tothe load store section by the functional units. This is true if thecorresponding segment base is zero. If the segment base is non-zero, thefunctional units send the logical address. This logical address whenadded to the segment base in the corresponding segment registergenerates the linear address. The buffer stores linear addresses andtherefore, the following scheme where an extra cycle is taken to computethe linear address is proposed.

For null segment indications from microcode and when all the segmentbases are zero, there is a bypass path to the load-store buffer from thefunctional units. This is shown in FIG. 46.

Floating Point Loads and Stores

The floating point instructions are done in microcode. Thus theload-store section handles the FP loads and stores as normal loads andstores. The unit load/store does not know that it is performing an FPload or store.

Special Registers (LSSPREG)

Special Registers in the load store section include the segmentregisters and the debug registers. The addresses that are generated bythe functional units do not account for the segment base. If theappropriate segment base is not zero, it has to be added to the resultgenerated by the functional units. In the case of non-zero segment base,an extra clock cycle is taken to compute the linear address. The LSSECmaintains all the segment registers and their invisible portion--the8-byte descriptor. The segment registers can be loaded using a loadinstruction with the selector value as the operand. The specialregisters are summarized below:

    ______________________________________                                        CS         Selector, CSDES.HI,                                                                            CSDES.LO                                          SS         Selector, SSDES.HI,                                                                            SSDES.LO                                          DS         Selector, DSDES.HI,                                                                            DSDES.LO                                          ES         Selector, ESDES.HI,                                                                            ESDES.LO                                          FS         Selector, FSDES.HI,                                                                            FSDES.LO                                          GS         Selector, GSDES.HI,                                                                            GSDES.LO                                          LDTR       Selector, LDTRDES.HI,                                                                          LDTRDES.LO                                        TR         Selector, TRDES.HI,                                                                            TRDES.LO                                          TEMP       Selector, TEMPDES.HI,                                                                          TEMPDES.LO                                        ______________________________________                                    

All of the above have BASE, LIMIT and ATTR fields within theirdescriptor HI and LO fields and can be read independently.

    ______________________________________                                        GDTR           BASE,        LIMIT                                             IDTR           BASE,        LIMIT                                             FPDP           FPDP.HI,     FPDP.LO                                           CAR                                                                           DR0                                                                           DR1                                                                           DR2                                                                           DR3                                                                           DR7                                                                           ______________________________________                                    

The SRB can access all the following fields: DESC.HI, DESC.LO, LIMIT,ATTR, BASE, SELECTOR.

Limit Checking

The limit checking is done in the LSSPREG section. The followinginformation is needed to perform limit checking.

Data size--byte, word, dword.

PE, VM bits.

D, G, ED from the descriptor.

The load-store buffer has the linear address, and the limit obtainedfrom the segment descriptor is a logical limit. In order to performlimit checking with these two quantities, the segment base is added tothe limit to obtain the linear limit. The linear limit is computed whenperforming the segment load and a copy of it stored for use during limitchecking. Protection checking logic is also in this block. Thelimit/protection check logicis pipelined, as shown in FIG. 47.

Handling Special Register Moves

It has been found that serialization in microcode of severalinstructions limits performance. To improve performance, someinstructions are not serialized in Processor 500. This requires hardwaresupport in the respective units.

All moves to and from GS, ES and FS are not serialized in dispatch.These moves will be dispatched to the LSSEC similar to a load or astore, but they sit in a special four-entry FIFO. This FIFO isphysically located in the LSSPREG block. The move-to's in this bufferare performed when they are non-speculative. This is very much similarto the store operation. FIG. 48 illustrates the details of each entry ofthis LSRBUF.

Segment Loads

A segment load instruction will be dispatched both to the LDSTBUF andthe special registers FIFO. These instructions are interpreted by theLDSTBUF as loads and as writes (or move to's) by the special registerFIFO. The results from the loads are forwarded into the FIFO whichwrites the appropriate segment registers after the FIROB indicates thatthe instruction is non-speculative.

In addition to the instruction type information, the decode/microcodeunit sends additional control bits to the load-store section directly assummarized below. The information is to inform the LSSEC which segmentis being accessed, the type of protection checks that need to beperformed, etc. That there are four buses INSLSnB 10:0! that supply thisinformation.

Also, the LSSEC needs three bits of information communicated by themicrocode about types of segment loads, types of protection checking,reporting of error codes, etc.

INSLSnB 10:8!

CS loads

000 Normal segment load for a far jump, call, move, etc.

001 due to selector from call gate when executing jmp instruction.

010 due to selector call from call gate when executing call instruction.

011 due to selector from task gate. Should not result in another gate.Should be type TSS

100 Selector from RET instruction.

101 Selector from IRET instruction.

110 Selector from INT instruction.

111 This is used when loading selectors from TSS on a task switch.

SS Loads

001 Selector from TSS. This is used when performing SS load on aprivilege level change using a selector that has been read out of TSS.

111 Report errors as TSS faults as selector came from TSS. DS, ES, ES,GS, LDTR, TR

111 Report errors as TSS faults as selector came from TSS.

TR Loads

001 All busy checks are done by microcode when performing task switches.However, when loading TR using LTR instruction, the busy check has to bedone by LSSEC. 000 This is the normal TR load. No busy checks. Whenperforming task switches.

General Loads and Stores

010 When these operations are performed, report CPL as zero to DCACHEand TLB so that they can do page level protection checking using CPL of0 instead of CURCPL.

001 All pushes and pops of copying parameters, when switching stacks,will report errors as TSS faults.

011 Ignore alignment checking.

110 Used in CMPXCHG8B routine.

IDT Loads

001 This switch indicates that the IDT lookup was initiated by asoftware interrupt and the DPL and CPL checks should be done.

010 When these operations are performed, report CPL as zero to DCACHEand TLB so that they can do page level protection checking using CPL of0 instead of CURCPL.

A diagram of protection checking hardware is illustrates in FIG. 49.

LSSEC--Cache master interface:

This section covers the following topics:

1. Protocol for handling data cache misses

2. Protocol for handling non-cacheable accesses

3. Protocol for handling locked accesses

Misses:

Load Miss Paging on, DCACHE on!

When a load access to the data cache misses, the LSSEC does aspeculative lookup of the TLB/PT to check if there is an alias. It doesthis by asserting LS₋₋ MMU₋₋ RD signal with the linear address on theDADDR 31:0! bus. The CMASTER can respond with the following signals

TLB₋₋ MISS--The look up in the TLB missed. Therefore, a true miss.

PT₋₋ MISS--The lookup hit in the TLB but missed in the PT. Therefore, atrue miss.

CM₋₋ DC₋₋ ALIAS--There is an alias in the data cache.

IC₋₋ CACHED--The line requested is in the instruction cache.

Load Miss--Data cache on, paging off

When paging is off, the linear address and the physical address are thesame. If a load accesses misses in the data cache, it is also going tomiss in the TLB. So, in this case, as soon as there is a data cachemiss, LSSEC goes external to get the data of interest. The M bit for theload is set when a dcache miss is encountered.

As soon as the instruction becomes non-speculative, the address is senton the DADDR bus and the signals LS₋₋ MMU₋₋ RD and LS₋₋ NOT₋₋ SP areasserted. At this point in time, the CMASTER and the DCACHE go throughthe reload sequence. After reload is done, the data cache asserts theDCDATRDY signal and in the next cycle the dcache drives the result onthe LSRES bus while the lssec drives the LSTAG bus.

Load Miss--Data Cache On, Paging On

In this case, when the load-store section encounters a data cache miss,it does a speculative TLB/PT access. It does so by asserting LS₋₋ MMU₋₋RD and driving the DADDR bus with the load linear address. It is notedthat LS₋₋ NOT₋₋ SP signal is not asserted during speculative lookups.During speculative look up, the CMASTER can assert one of the followingthree signals--TLB LS₋₋ MISS access missed in the TLB!, CM₋₋ DC₋₋ ALIASaccess hit in the TLB and the physical tags reported an alias!, PT₋₋MISS access hit in the TLB but missed in the physical tags!, MMULSPGFLTencountered a protection violation in the TLB!. For the case of CM₋₋DC₋₋ ALIAS, the CMASTER sends the column information to the data cacheand the LSSEC drives the address and LSUPDATERD signal to the data cachewhich will then update its tag and drive the result. For TLB₋₋ LS₋₋ MISSand PT₋₋ MISS cases, the M bit in the entry is set. When the instructionbecomes non-speculative, the address is sent on the DADDR bus and thesignals LS₋₋ MMU₋₋ RD and LS₋₋ NOT₋₋ SP are asserted. It is now theresponsibility of the CMASTER to resolve TLB miss and get the line ofinterest from memory. For the case of page fault, the LSSEC sends a pagefault status to the FIROB with a page fault vector on the result bus. Ifthe instruction is non-speculative, the FIROB then asserts EXCEPTION anddrives the entry point of the page fault routine.

Handling Physical Addresses from Microcode

The microcode can send physical addresses for HDT mode, microcode patch,etc. The LSSEC and the cache master handles this. There is an indicationon the INSLS bus if an address from the functional unit is a physicaladdress. The LSSEC latches this information in the unified load-storebuffer. It treats this access as a non-cacheable access. The address isdriven on the DADDR bus to the CMASTER and the signal LS₋₋ PHYSICAL isasserted. The cache master, when it sees this signal, disablestranslation and performs a physical tags look up. If there is an alias,the CMASTER asserts CM₋₋ DC₋₋ ALIAS signal and drives the columninformation over to the data cache. After the data cache signals that itis ready to receive the access, the LSSEC drives the index withoutLSUPDATERD being asserted. The data cache then drives the data andpresents the hit status to the functional units and FIROB.

If there is no alias, the access is treated as a non-cacheable access.

There is a problem with this approach--load-store dependency checking isdisabled when there is a combination of physical and linear addresses.But, this may be handled if only linear addresses and only physicaladdresses are used. The best solution is if microcode introducesserialization when switching between HDT (patch) and non-HDT (non-patch)modes. Dependency checking can now be handled in the same way for allthe modes.

SIGNAL LIST

INPUTS

    ______________________________________                                        BIU.sub.-- NC 2:0!                                                                          Input from CMASTER                                              ______________________________________                                    

When this signal is asserted, the reload data should not be placed inthe data cache.

    ______________________________________                                        CANENTRY       Input from FIROB                                                                           ICLK7                                             ______________________________________                                    

When this bit is asserted, the instruction pointed to by WRPTR isinvalidated.

    ______________________________________                                        CANENTRY1      Input from FIROB                                                                           ICLK8                                             ______________________________________                                    

When this bit is asserted, the instruction pointed to by WRPTR1 isinvalidated.

    ______________________________________                                        CMSTRBUSY     Input from CMASTER                                              ______________________________________                                    

This signal indicates that the cache master is busy.

    ______________________________________                                        CM.sub.-- DC.sub.-- ALIAS                                                                    Input from CMASTER                                             ______________________________________                                    

This signal indicates that there is an alias in the data cache for therequest sent.

    ______________________________________                                        CR0WP                                                                         CR0AM                                                                         CR0PE        Inputs from SRB                                                  ______________________________________                                    

Bits from the CR0 register. WP bit is used in preventing supervisoryaccesses to read-only pages. The AM flag if set means that all unalignedaccesses will generate exceptions. The PE bit is used to change the modeof processor to protected mode from real mode.

    ______________________________________                                        DCCNCLA       Input from DCACHE                                                                           ICLK7                                             ______________________________________                                    

This signal indicates that the port A access must be canceled andreissued in the next clock cycle.

    ______________________________________                                        DCCNCLB       Input from DCACHE                                                                           ICLK7                                             ______________________________________                                    

This signal indicates that the port B access must be canceled andreissued in the next clock cycle.

    ______________________________________                                        DCDATARDY     Input from DCACHE                                                                           ICLK13                                            DCBUSY        Input from DCACHE                                                                           ICLK12                                            ______________________________________                                    

The data cache asserts this signal to inform that it is busy eitherdoing a snoop read, snoop invalidation, reload or a cache line read thatis done prior to a reload. The LSSEC does not send any access as long asthis is asserted.

    ______________________________________                                        DCFINRLD      Input from DCACHE                                                                           ICLK13                                            ______________________________________                                    

Indication from the data cache that a reload has been completed. TheLSSEC can now request for DCACHE loads and stores.

    ______________________________________                                        DCLSD  1:0!   Input from DCACHE                                                                           ICLK5                                             ______________________________________                                    

DCLSD 0! and DCLSD 1! are the dirty bits corresponding to port A andport B respectively. The are used by the LSSEC during stores todetermine if the corresponding dirty bit in the page table entry is setcorrectly in the TLB.

    ______________________________________                                        DCLSPROK  1:0! Input from DCACHE                                                                           ICLK5                                            ______________________________________                                    

Indicates that a protection violation occured during a DCACHE access.Bit 0 corresponds to port A while bit 1 corresponds to port B.

    ______________________________________                                        DCPAPRHIT     Input from DCACHE                                                                           ICLK3                                             ______________________________________                                    

Indication that the port A access hit in the predicted way.

    ______________________________________                                        DCPBPRHIT     Input from DCACHE                                                                           ICLK3                                             ______________________________________                                    

Indication that the port B access hit in the predicted way.

    ______________________________________                                        DCUNPAHIT     Input from DCACHE                                                                           ICLK3                                             ______________________________________                                    

Indication that the port A access hit in one of the unpredicted ways.

    ______________________________________                                        DCUNPBHIT     Input from DCACHE                                                                           ICLK3                                             ______________________________________                                    

Indication that the port B access hit in one of the unpredicted ways.

    ______________________________________                                        EFIOPL  1:0!                                                                  EFLAGSAC                                                                      EFLAGSVM                                                                      EFLAGSRF    Inputs from FIROB  EFLAGS!                                                                      ICLK5                                           ______________________________________                                    

Various bits from the EFLAGS register that are needed elsewhere in theprocessor. AC is the alignment check bit used in preventing unalignedmemory accesses if AM is set in CR0. The VM bit is for virtual mode ofoperation. The RF flag is looked up by the break point detectionhardware before reporting any breakpoint matches. If RF is set,breakpoint matches are not reported. IOPL bits are used for input/outputprotection checking.

    ______________________________________                                        EXCEPTION      Input from FIROB                                                                           ICLK5                                             ______________________________________                                    

Global exception indication from ROB to cancel all pending instructionsand to not return any more results.

    ______________________________________                                        IC.sub.-- CACHED                                                                            Input from CMASTER                                              ______________________________________                                    

Indication that the line the LSSEC/DCACHE requested is in the ICACHE.

    ______________________________________                                        INSLS0B  10:0!  Input from FU                                                 INSLS1B  10:0!  Input from FU                                                 INSLS2B  10:0!  Input from FU                                                 INSLS3B  10:0!  Input from FU                                                                            ICLK3                                              ______________________________________                                    

These buses give the load-store section information as to which segmentis currently being accessed and types of protection checking that needto be performed. The information is sent to the FU by decode/microcodeand the FU sends the information while computing the linear address.

    ______________________________________                                        IRESET      Input from CLOCKS                                                 ______________________________________                                    

Global reset signal. Clears all LDSTBUF entries. Puts the control statemachine to idle/reset state.

    ______________________________________                                        ITYPE0B  1:0! Input from IDECODE                                              ITYPE1B  1:0! Input from IDECODE                                              ITYPE2B  1:0! Input from IDECODE                                              ITYPE3B  1:0! Input from IDECODE                                              ICLK3                                                                         ______________________________________                                    

These buses give the type of instructions dispatched.

    ______________________________________                                        00               NULL                                                         01               LOAD                                                         10               STORE                                                        11               LOAD-OP-STORE                                                LINEPTR  2:0! Input from FIROB                                                                           ICLK3                                              ______________________________________                                    

These buses give the FIROB line number associated with the instructionsdispatched.

    ______________________________________                                        RES0B  31:0!   Input from FU                                                  RES1B  31:0!   Input from FU                                                  RES2B  31:0!   Input from FU                                                  RES3B  31:0!   Input from FU                                                  ICLK12                                                                        ______________________________________                                    

The store data is driven on the result buses by the functional units.

    ______________________________________                                        RESLA0B  14:0!  Input from FU                                                 RESLA1B  14:0!  Input from FU                                                 RESLA2B  14:0!  Input from FU                                                 RESLA3B  14:0!  Input from FU                                                                            ICLK10                                             ______________________________________                                    

Linear addresses from the functional units. The result buses are slow.The linear addresses need to come to the LSSEC faster so that it canperform two accesses at the end of the cycle. If needed, the bus widthcan be reduced to 14 bits--so that the cache indexing can be done. Therest of the bits can be grabbed from the result buses.

    ______________________________________                                        RSTAT0LS  2:0!  Input from FU                                                 RSTAT1LS  2:0!  Input from FU                                                 RSTAT2LS  2:0!  Input from FU                                                 RSTAT3LS  2:0!  Input from FU                                                                            ICLK3                                              ______________________________________                                    

    ______________________________________                                        00       Null                                                                 01       Address is driven by the functional unit on the                               RESLA bus                                                            10       Data is driven on the result bus by the functional                            unit                                                                 ______________________________________                                    

Bit 2=1 indicates that the linear address generated is for a loadinstruction. This is used by the LSSEC to forward linear addresses tothe data cache directly to the functional units.

    ______________________________________                                        RTAG0B  2:0!    Input from FU                                                 RTAG0B  2:0!    Input from FU                                                 RTAG0B  2:0!    Input from FU                                                 RTAG0B  2:0!    Input from FU                                                                            ICLK3                                              ______________________________________                                    

This gives the ROB line number of the instruction that the FU isprocessing.

    ______________________________________                                        TLB.sub.-- LS.sub.-- MISS                                                                    Input from CMASTER                                             ______________________________________                                    

This is the TL hit/miss indication from MMU.

    ______________________________________                                        WRPTR  5:0!   Input from FIROB                                                                           ICLK5                                              ______________________________________                                    

This gives the line and entry numbers of the instruction that is readyto get retired.

    ______________________________________                                        WRPTR1  5:0!  Input from FIROB                                                                           ICLK6                                              ______________________________________                                    

This gives the line and entry numbers of the next store in a FIROB linethat can be retired with another store. This aids in sending out twostore accesses.

OUTPUTS

    ______________________________________                                        CURCPL  1:0!  Output to DCACHE                                                                           ICLK10                                             ______________________________________                                    

This indicates the current privilege level. The data cache uses the D,R/W and U/S bits in conjunction with the CURCPL information to determinethe protection violations.

    ______________________________________                                        LDORST  1:0!  Output to DCACHE                                                                           ICLK12                                             ______________________________________                                    

Indication whether an access is a load or a store.

    ______________________________________                                        LSBNKCT       Output to DCACHE                                                                           ICLK13                                             ______________________________________                                    

This indicates if there is a bank conflict for the two concurrent datacache accesses to be performed.

    ______________________________________                                        LSBREAK  1:0!    Output to FIROB                                                                           ICLK13                                           ______________________________________                                    

Load/Store breakpoint identification from LSSEC. When a load or storebreakpoint status is returned, this 2-bit code indicates which of thefour debug address registers had the match.

    ______________________________________                                        LSCNT  2:0!  Output to IDECODE                                                ICLK11                                                                        ______________________________________                                    

Number of free entries in the LDSTBUF. Decode can make use of thisinformation and dispatch instructions accordingly.

    ______________________________________                                        LSDCEXC  1:0!  Output to DCACHE                                                                           ICLK13                                            ______________________________________                                    

This signal indicates the data cache that limit check or protectionviolation has occured on the current port A bit0! or port B bit1!access. If the access is a store, the data cache has to restore the databack into the data cache.

    ______________________________________                                        LS.sub.-- FAKE.sub.-- LOAD                                                                   Output to CMASTER                                                                           ICLK5                                            ______________________________________                                    

    ______________________________________                                        LS.sub.-- FAKE.sub.-- LOCK                                                                   Output to CMASTER                                                                           ICLK5                                            LSINDXCT       Output to DCACHE                                                                            ICLK14                                           ______________________________________                                    

This indicates if there is an index conflict for the two concurrent datacache accesses to be performed.

    ______________________________________                                        LS.sub.-- IO.sub.-- CYCLE                                                                    Output to CMASTER                                                                           ICLK5                                            LSLINAD0  31:2!                                                                              Output to DCACHE                                                                            ICLK14                                           ______________________________________                                    

The linear address which would be sent as the port A access to the datacache.

    ______________________________________                                        LSLINAD1  31:2! Output to DCACHE                                                                           ICLK14                                           ______________________________________                                    

The linear address which would be sent as the port B access to the datacache.

    ______________________________________                                        LS.sub.-- LOCK Output to CMASTER                                                                           ICLK5                                            LS.sub.-- MMU.sub.-- RD                                                                      Output to CMASTER                                                                           ICLK5                                            ______________________________________                                    

This signal is asserted when the load-store section is performing a datacache read. The read is done speculatively.

    ______________________________________                                        LS.sub.-- UNLOCK                                                                             Output to CMASTER                                                                           ICLK5                                            LS.sub.-- MMU.sub.-- WR                                                                      Output to CMASTER                                                                           ICLK5                                            ______________________________________                                    

This signal is asserted when the load-store section is performing a datacache write. The look up is done speculatively.

    ______________________________________                                        LSPASHFT  1:0! Output to DCACHE                                                                           ICLK14                                            ______________________________________                                    

This signal gives the shift count to the data cache when performingunaligned accesses within a double word on port A.

    ______________________________________                                        00                 shft by 0                                                  01                 shft by 1 byte                                             10                 shft by 2 bytes                                            11                 shft by 3 bytes                                            LSPBSHFT  1:0! Output to DCACHE                                                                           ICLK14                                            ______________________________________                                    

This signal gives the shift count to the data cache when performingunaligned accesses within a double word on port B.

    ______________________________________                                        LSRES0B  31:0!/XLSRES0                                                                         Output to FNCU/FIROB                                         ICLK13                                                                        ______________________________________                                    

Result bus returning results for AC0. The intermediate results forload-op-store instructions are indicated by the LSSTAT0B bus.

XLSRES0B is the inverse of LSRES0B. The differential buses are used forspeed.

    ______________________________________                                        LSRES1B  31:0!/XLSRES1                                                                         Output to FNCU/FIROB                                         ICLK13                                                                        ______________________________________                                    

Result bus returning results for AC1. The intermediate results forload-op-store instructions are indicated by the LSSTAT1B bus.

    ______________________________________                                        LST.sub.-- SRB                                                                LST.sub.-- LDOP                                                               LST.sub.-- LOAD                                                               LSSTAT0B  3:0!                                                                              Output to FIROB/FNCU                                                                         ICLK10                                           ______________________________________                                    

Status of the result returned on LSRES0B. Bit 3 defines whether a resultis intermediate.

    ______________________________________                                        LSSTAT1B  3:0!                                                                              Output to FIROB/FNCU                                                                         ICLK10                                           ______________________________________                                    

Status of the result returned on LSRES1B. Bit 3 defines whether a resultis intermediate.

    ______________________________________                                        LSTAG0B  5:0! Output to FNCU/FIROB                                                                         ICLK5                                            ______________________________________                                    

This gives the tag of the instruction returned on LSRES0B.

    ______________________________________                                        LSTAG1B  5:0! Output to FNCU/FIROB                                                                         ICKL5                                            ______________________________________                                    

This gives the tag of the instruction returned on LSRES1B.

    ______________________________________                                        LSUNALGN  1:0!  Output to DCACHE                                                                           ICLK13                                           ______________________________________                                    

LSUNALGN 0! indicates that the load/store access to the data cache fromport A is unaligned. LSUNALGN 1! indicates the same for port B. Anunaligned load can be issued to either port A or port B whereas anunaligned store uses both ports simultaneously.

    ______________________________________                                        LSUPDATERD     Output to DCACHE                                                                           ICLK10                                            ______________________________________                                    

This signal indicates the data cache to update its tag with the newlinear address. The data cache also needs to update its valid and statusbits.

    ______________________________________                                        LSWYPRCT      Output to DCACHE                                                                           ICLK14                                             ______________________________________                                    

This indicates if there is an index conflict in the way prediction arrayfor the two concurrent accesses to be performed.

    ______________________________________                                        PABYTEN  3:0!  Output to DCACHE                                                                           ICLK14                                            ______________________________________                                    

The byte enables saying which of the bytes in a bank are being accessed.port

    ______________________________________                                        PBBYTEN  3:0!  Output to DCACHE                                                                           ICLK14                                            ______________________________________                                    

The byte enables saying which of the bytes in a bank are being accessed.port

    ______________________________________                                        SBADAT  31:0!  Output to DCACHE                                                                           ICLK13                                            ______________________________________                                    

This bus is used to drive the store data for port A. This is a staticbus.

    ______________________________________                                        SBBDAT  31:0!  Output to DCACHE                                                                           ICLK13                                            ______________________________________                                    

This bus is used to drive the store data for port B.

    ______________________________________                                        ST.sub.-- NC Output to CMASTER                                                                           ICLK5                                              ______________________________________                                    

This signal indicates the CMASTER that the store that just got retiredis a non-cacheable store.

    ______________________________________                                        SUPERV                                                                        CS32X16                                                                       SS32X16      Outputs to IDECODE                                                                          ICLK10                                             ______________________________________                                    

BIDIRECTS

    ______________________________________                                        DADDR  31:0!                                                                              Birdirect. Connects to DCACHE and                                        CMASTER   ICLK13                                                       ______________________________________                                    

This bus is used by the load-store section to send linear address to thecache master to check for aliases, initiate reloads and writebacks. Thecache master sends the new tag on this bus to the data cache for thecase of an alias.

    ______________________________________                                        DATB  63:0! Birdirect. Connects to DCACHE and                                        CMASTER   ICLK13                                                       ______________________________________                                    

This shared bus is used to send data to the CMASTER for non-cacheableand write through stores. The data cache and the cache master use thisbus for reloads and writebacks.

    ______________________________________                                        SRBB  31:0!    Bidirect - SRB                                                                           ICLK14                                              ______________________________________                                    

This bi-directional bus is used by the SRB for moving to and fromspecial registers and accessing arrays.

Partitioning of LSSEC

The load-store section is partitioned into the following blocks:

1. LDSTDAT--The store data array.

2. LDSTADR--The array having the address portion of the load-storebuffer. It also contains the dependency checking logic for store dataforwarding.

3. LDSTSTAT--Array holding the status information. The statusinformation is looked at mostly by the control unit.

4. LDSTTAGS--Array containing the instruction tags. This array isupdated by the decode unit and the information is needed to updateLDSTDAT and LDSTADR blocks and to perform dependency checking.

5. LSSPREG--Array of segment registers. Further details of this blockare provided above.

6. LSCTL--The load-store section control block.

A block diagram of this partitioning is shown as FIG. 50.

Layout of LSSEC

FIG. 51 shows a possible layout configuration of the LSSEC. LDSTDAT,LDSTADR, LDSTTAGS and LDSTSTAT constitute the unified load-store buffer(LDSTBUF).

LDSTDAT (The array containing the store data)

This array contains the store data. There are eight entries of 32-bitseach. The sources of updates to this array are the functional units. Thearray looks at the LATYPE signal which indicates whether address or datais driven on the result bus. The control section indicates which of theentries need to be updated (LSUPD 15:0!). LSRDDAT0 15:0! and LSRDDAT115:0! indicate which of the two entries go out as accesses for stores.LSFWD0 15:0! LSFWD1 15:0! indicate as to which entries need to getforwarded over to LSRES0 and LSRES1 buses.

Signal List

    ______________________________________                                        RES0B  31:0!                                                                  RES1B  31:0!                                                                  RES2B  31:0!                                                                  RES3B  31:0! Inputs from Functional Units                                     ______________________________________                                    

The result buses from the functional units.

    ______________________________________                                        RSTAT0LS  2:0!  Input from FU                                                 RSTAT1LS  2:0!  Input from FU                                                 RSTAT2LS  2:0!  Input from FU                                                 RSTAT3LS  2:0!  Input from FU                                                                            ICLK3                                              00       Null                                                                 01       Address is driven by the functional unit on the                               RESLA bus                                                            10       Data is driven on the result bus by the functional                            unit                                                                 ______________________________________                                    

Bit 2=1 indicates that the linear address generated is for a loadinstruction. This is used by the LSSEC to forward linear addresses tothe data cache directly to the functional units.

    ______________________________________                                        LS0UPD  3:0!                                                                  LS1UPD  3:0!                                                                  LS2UPD  3:0!                                                                  LS3UPD  3:0!                                                                  LS4UPD  3:0!                                                                  LS5UPD  3:0!                                                                  LS5UPD  3:0!                                                                  LS6UPD  3:0!                                                                  LS7UPD  3:0!  Inputs from LDSTTAGS                                            ______________________________________                                    

The update signals after comparing the tags.

    ______________________________________                                        LS1RDDAT  7:0! Input from LSCTL                                               ______________________________________                                    

Indicates which of the entries must go out as AC0 for stores.

    ______________________________________                                        LS0FWD  7:0!  Input from LDSTADR                                              ______________________________________                                    

Indicates which of the entries must go out as AC1 for stores.

    ______________________________________                                        LS1FWD  7:0!  Input from LDSTADR                                              ______________________________________                                    

Indicates which of the entries need to be forwarded on to LSRES0.

    ______________________________________                                        LS0RDDAT 7:0!      Input from LSCTL                                           ______________________________________                                    

Indicates which of the entries need to be forwarded on to LSRES1.

    ______________________________________                                        LSRES0  31:0!, XLSRES0  31:0!                                                                    Output to FUn/FIROB                                        ______________________________________                                    

Load-store result bus.

    ______________________________________                                        LSRES1  31:0!, XLSRES1  31:0!                                                                    Output to FUn/FIROB                                        ______________________________________                                    

Load-store result bus.

    ______________________________________                                        SHF1ENT  7:0!                                                                 SHF2ENT  7:0!  Inputs from LSCTL                                              ______________________________________                                    

Shift signals to shift the entries down.

shf1ent xor SHF2ENT=1--shift entry by 1

shf1ent and SHF2ENT=1--shift entry by 2

    ______________________________________                                        DATB  63:0!                                                                              Birdirect. Connects to DCACHE and CMASTER                          ______________________________________                                    

This shared bus is used to send data to the CMASTER for non-cacheableand write through stores. The data cache and the cache master use thisbus for reloads and writebacks.

    ______________________________________                                        SBADAT  31:0! Output to the data cache                                        ______________________________________                                    

This bus is used to drive the store data for port A. This is a staticbus.

    ______________________________________                                        SBBDAT  31:0! Output to the data cache                                        ______________________________________                                    

This bus is used to drive the store data for port B.

LDSTADR (The array containing the load-store address)

This array of eight entries contains the address for the load-storeaccesses. The update sources for the array are the functional units. Thefunctional units send the bits 14:0! on the RESLAn buses and the entireaddress on the RESnB buses. The array gets updated via the RESnB buses.The RESLA buses are much faster than the result buses and are used tosend out the index for the data cache accesses as soon as possible.Also, the LSSEC needs to determine bank conflicts and unalignedaccesses. This again needs the LSBs of the address available to theLSSEC early.

This block also contains the logic for dependency checking againststores in the buffer and sending the forwarding signals to LDSTDAT.

Signal List

    ______________________________________                                        RES0B  31:0!                                                                  RES1B  31:0!                                                                  RES2B  31:0!                                                                  RES3B  31:0! Inputs from Functional Units                                     ______________________________________                                    

The result buses from the functional units.

    ______________________________________                                        RSTAT0LS  2:0!  Input from FU                                                 RSTAT1LS  2:0!  Input from FU                                                 RSTAT2LS  2:0!  Input from FU                                                 RSTAT3LS  2:0!  Input from FU                                                                            ICLK3                                              00       Null                                                                 01       Address is driven by the functional unit on the                               RESLA bus                                                            10       Data is driven on the result bus by the functional                            unit                                                                 ______________________________________                                    

Bit 2=1 indicates that the linear address generated is for a loadinstruction. This is used by the LSSEC to forward linear addresses tothe data cache directly to the functional units.

    ______________________________________                                        LS0UPD  3:0!                                                                  LS1UPD  3:0!                                                                  LS2UPD  3:0!                                                                  LS3UPD  3:0!                                                                  LS4UPD  3:0!                                                                  LS4UPD  3:0!                                                                  LS5UPD  3:0!                                                                  LS6UPD  3:0!                                                                  LS7UPD  3:0!  Inputs from LDSTTAGS                                            ______________________________________                                    

The update signals after comparing the tags.

    ______________________________________                                        LSLINAD0  31:0! Output to DCACHE                                              ______________________________________                                    

The address for access AC0.

    ______________________________________                                        LSLINAD1  31:0! Output to DCACHE                                              ______________________________________                                    

The address for access AC1.

    ______________________________________                                        SHF1ENT  7:0!                                                                 SHF2ENT  7:0!  Inputs from LSCTL                                              ______________________________________                                    

Shift signals to shift the entries down.

shf1ent xor SHF2ENT=1--shift entry by 1

shf1ent and SHF2ENT=1--shift entry by 2

    ______________________________________                                        LS0RDDAT  7:0! Input from LSCTL                                               ______________________________________                                    

Indicates which of the entries must go out as AC0 for stores.

    ______________________________________                                        LS1RDDAT  7:0! Input from LSCTL                                               ______________________________________                                    

Indicates which of the entries must go out as AC1 for stores.

    ______________________________________                                        LS0FWD  7:0!  Input from LDSTADR                                              ______________________________________                                    

Indicates which of the entries need to be forwarded on to LSRES0.

    ______________________________________                                        LS1FWD  7:0!  Input from LDSTADR                                              ______________________________________                                    

Indicates which of the entries need to be forwarded on to LSRES1.

LDSTSTAT (The array containing the control/status information)

This array is also eight entries deep and contains the control/statusinformation of the loads and stores in the LSSEC. The update source forthis array is the load-store section itself.

Signal List

    ______________________________________                                        ALL0ENT  2:0!                                                                 ALL1ENT  2:0!                                                                 ALL2ENT  2:!                                                                  ALL3ENT  2:0!  Inputs from LSCTL                                              ______________________________________                                    

These indicate what entries can be allocated for the next dispatch.

    ______________________________________                                        ALL0V  3:0!                                                                   ALL1V  3:0!                                                                   ALL2V  3:0!                                                                   ALL3V  2:0!   Inputs from LSCTL                                               ______________________________________                                    

These signals are associated with ALLnENT and indicate which of the fourdispatch positions they correspond to.

    ______________________________________                                        SET0MISS  2:0! Input from LSCTL                                               ______________________________________                                    

Set the M bit for the entry indicated by SET0MISS if a port A accessmissed.

    ______________________________________                                        SET1MISS  2:0! Input from LSCTL                                               ______________________________________                                    

Set the M bit for the entry indicated by SET1MISS if a port B accessmissed.

    ______________________________________                                        SETM2  2:0!   Input from LSCTL                                                ______________________________________                                    

Based on this, the M2 bit for an entry in the array is set.

    ______________________________________                                        SET0DEP  2:0! Input from LSCTL                                                ______________________________________                                    

Set the D (dependent) bit.

    ______________________________________                                        SET1DEP  2:0! Input from LSCTL                                                ______________________________________                                    

Set the D (dependent) bit.

    ______________________________________                                        SET0SER  2:0! Input from LSCTL                                                ______________________________________                                    

Set the S (Serialize) bit.

    ______________________________________                                        SET1SER  2:0! Input from LSCTL                                                ______________________________________                                    

Set the S (Serialize) bit.

    ______________________________________                                        LDAC  7:0!    Output to LSCTL                                                 ______________________________________                                    

Indicates which of the entries have valid loads that can go out asaccesses.

    ______________________________________                                        LSAC  1:0!    Output to LSCTL                                                 ______________________________________                                    

Indicates which of the entries have valid stores that can go out asaccesses.

    ______________________________________                                        CLRDEP       Input from LSCTL                                                 ______________________________________                                    

Clear the D bit for all the entries.

    ______________________________________                                        CLRMISS1      Input from LSCTL                                                ______________________________________                                    

Clear the M bit for all the entries.

    ______________________________________                                        SETNF       Input from LSCTL                                                  ______________________________________                                    

Set the NF bit for all the stores in the buffer. This will preventforwarding store data for dependent loads.

    ______________________________________                                        SHF1ENT  7:0!                                                                 SHF2ENT  7:0!  Inputs from LSCTL                                              ______________________________________                                    

Shift signals to shift the entries down.

shf1ent xor SHF2ENT=1--shift entry by 1

shf1ent and SHF2ENT=1--shift entry by 2

LDSTTAGS (Array containing the FIROB tags)

This 8-entry array contains the FIROB tags for the instructions in theLSSEC. The tags are looked up by the control unit during accessprioritization. The tags in the entries are compared against the tagsfrom the functional units when updating the address and data arrays. Thetags information is also needed when performing dependency checking.

Signal List

    ______________________________________                                        RLINE  2:0!   Input from FIROB                                                ______________________________________                                    

These buses give the FIROB line number associated with the instructionsdispatched.

    ______________________________________                                        RTAG0B  2:0!   Input from FUn                                                 RTAG0B  2:0!   Input from FUn                                                 RTAG0B  2:0!   Input from FUn                                                 RTAG0B  2:0!   Input from FUn                                                 ______________________________________                                    

This gives the ROB line number of the instruction that the FU isprocessing.

    ______________________________________                                        LS0UPD  3:0!                                                                  LS1UPD  3:0!                                                                  LS2UPD  3:0!                                                                  LS3UPD  3:0!                                                                  LS4UPD  3:0!                                                                  LS5UPD  3:0!                                                                  LS6UPD  3:0!                                                                  LS7UPD  3:0!                                                                              Output to LDSTDAT, LDSTADR and                                    LDSTSTAT.                                                                     ______________________________________                                    

The update signals after comparing the tags.

    ______________________________________                                        LSTAG0B  5:0! Output to FNCU/FIROB                                            ______________________________________                                    

This gives the tag of the instruction returned on LSRES0B.

    ______________________________________                                        LSTAG1B  5:0! Output to FNCU/FIROB                                            ______________________________________                                    

This gives the tag of the instruction returned on LSRES1B.

    ______________________________________                                        LS0RDDAT  7:0! Input from LSCTL                                               ______________________________________                                    

Indicates which of the entries must go out as AC0 for stores.

    ______________________________________                                        LS1RDDAT  7:0! Input from LSCTL                                               ______________________________________                                    

Indicates which of the entries must go out as AC1 for stores.

    ______________________________________                                        LS0FWD  7:0!  Input from LDSTADR                                              ______________________________________                                    

Indicates which of the entries need to be forwarded on to LSRES0.

    ______________________________________                                        LS1FWD  7:0!  Input from LDSTADR                                              ______________________________________                                    

Indicates which of the entries need to be forwarded on to LSRES1.

    ______________________________________                                        ALL0ENT  2:0!                                                                 ALL1ENT  2:0!                                                                 ALL2ENT  2:0!                                                                 ALL3ENT  2:0!  Inputs from LSCTL                                              ______________________________________                                    

These indicate what entries can be allocated for the next dispatch.

    ______________________________________                                        ALL0V  3:0!                                                                   ALL1V  3:0!                                                                   ALL2V  3:0!                                                                   ALL3V  3:0!   Inputs from LSCTL                                               ______________________________________                                    

These signals are associated with ALLnENT and indicate which of the fourdispatch positions they correspond to.

    ______________________________________                                        WRPTR        Input from FIROB                                                 ______________________________________                                    

This is the indication of head of the ROB.

    ______________________________________                                        WRPTR1       Input from FIROB                                                 ______________________________________                                    

This indicates the ROB position of the next non-speculative storeinstruction.

LSCTL (The control block for LSSEC)

Signal List

    ______________________________________                                        LS0RDDAT  2:0!                                                                            Output to LDSTDAT/LDSTADDR/LDSTTAGS                               ______________________________________                                    

Indicates which of the entries must go out as AC0 for stores.

    ______________________________________                                        LS1RDDAT  2:0!                                                                            Output to LDSTDAT/LDSTADDR/LDSTTAGS                               ______________________________________                                    

Indicates which of the entries must go out as AC1 for stores.

    ______________________________________                                        ALL0ENT  2:0!                                                                 ALL1ENT  2:0!                                                                 ALL2ENT  2:0!                                                                 ALL3ENT  2:0!                                                                              Outputs to LDSTTAGS/LDSTSTAT                                     ______________________________________                                    

These indicate what entries can be allocated for the next dispatch.

    ______________________________________                                        ALL0V  3:0!                                                                   ALL1V  3:0!                                                                   ALL2V  3:0!                                                                   ALL3V  3:0!                                                                              Outputs to LDSTTAGS/LDSTSTAT                                       ______________________________________                                    

These signals are associated with ALLnENT and indicate which of the fourdispatch positions they correspond to.

    ______________________________________                                        SET0MISS  2:0! Output to LDSTSTAT                                             ______________________________________                                    

Set the M bit for the entry indicated by SET0MISS if a port A accessmissed.

    ______________________________________                                        SET1MISS  2:0! Output to LDSTSTAT                                             ______________________________________                                    

Set the M bit for the entry indicated by SET1MISS if a port B accessmissed.

    ______________________________________                                        SETM2  2:0!  Output to LDSTSTAT                                               ______________________________________                                    

Based on this, the M2 bit for an entry in the array is set.

    ______________________________________                                        SET0DEP  2:0! Output to LDSTSTAT                                              ______________________________________                                    

Set the D (dependent) bit.

    ______________________________________                                        SET1DEP  2:0! Output to LDSTSTAT                                              ______________________________________                                    

Set the D (dependent) bit.

    ______________________________________                                        SET0SER  2:0! Output to LDSTSTAT                                              ______________________________________                                    

Set the S (Serialize) bit.

    ______________________________________                                        SET1SER  2:0! Output to LDSTSTAT                                              ______________________________________                                    

Set the S (Serialize) bit.

    ______________________________________                                        LDAC  7:0!   Input from LDSTSTAT                                              ______________________________________                                    

Indicates which of the entries have valid loads that can go out asaccesses.

    ______________________________________                                        LSAC  1:0!   Input from LDSTSTAT                                              ______________________________________                                    

Indicates which of the entries have valid stores that can go out asaccesses.

    ______________________________________                                        CLRDEP       Output to LDSTSTAT                                               ______________________________________                                    

Clear the D bit for all the entries.

    ______________________________________                                        CLRMISS1     Output to LDSTSTAT                                               ______________________________________                                    

Clear the M bit for all the entries.

    ______________________________________                                        SETNF       Output to LDSTSTAT                                                ______________________________________                                    

Set the NF bit for all the stores in the buffer. This will preventforwarding store data for dependent loads.

    ______________________________________                                        SHF1ENT  7:0!                                                                 SHF2ENT  7:0!   Output to                                                     ______________________________________                                    

LDSTDAT/LDSTADDR/LDSTTAGS/LDSTSTAT

Shift signals to shift the entries down.

shf1ent xor SHF2ENT=1--shift entry by 1

shf1ent and SHF2ENT=1--shift entry by 2

LSSPREG (The Load-Store Special Registers)

Signal List

    ______________________________________                                        LSRES0 31:0!, XLSRE0 31:0!                                                                         Input from LDSTDAT                                       ______________________________________                                    

Load-store result bus.

    ______________________________________                                        LSRES1 31:0!, XLSRES1 31:0!                                                                        Input from LDSTDAT                                       ______________________________________                                    

Load-store result bus.

    ______________________________________                                        IRESET           Input from CLOCKS                                            ______________________________________                                    

Global reset signal. Clears all LDSTBUF entries. Puts the control statemachine to idle/reset state.

    ______________________________________                                        INSLS0B 10:0!  Input from FU                                                  INSLS1B 10:0!  Input from FU                                                  INSLS2B 10:0!  Input from FU                                                  INSLS3B 10:0!  Input from FU ICLK3                                            ______________________________________                                    

These buses give the load-store section information as to which segmentis currently being accessed and types of protection checking that needto be performed. The information is sent to the FU by decode/microcodeand the FU sends the information while computing the linear address.

    ______________________________________                                        ITYPE0B 1:0!  Input from IDECODE                                              ITYPE1B 1:0!  Input from IDECODE                                              ITYPE2B 1:0!  Input from IDECODE                                              ITYPE3B 1:0!  Input from IDECODE                                                                             ICLK3                                          ______________________________________                                        These buses give the type of instructions dispatched.                         00               NULL                                                         01               LOAD                                                         10               STORE                                                        11               LOAD-OP-STORE                                                ______________________________________                                        LINEPTR 2:0!  Input from FIROB ICLK3                                          ______________________________________                                    

These buses give the FIROB line number associated with the instructionsdispatched.

    ______________________________________                                        LSRLMODE      Input from LSCTL                                                                              ICLK3                                           ______________________________________                                    

Indicates that the processor is in real mode.

    ______________________________________                                        LSPRMODE      Input from LSCTL                                                                              ICLK3                                           ______________________________________                                    

Indicates that the processor is in protected mode.

    ______________________________________                                        LSEFLAGSVM     Input from LSCTL                                                                              ICLK3                                          ______________________________________                                    

Indicates that the processor is in V86 mode.

    ______________________________________                                        RES0B 31:0!    Input from FU                                                  RES1B 31:0!    Input from FU                                                  RES2B 31:0!    Input from FU                                                  RES3B 31:0!    Input from FU ICLK12                                           ______________________________________                                    

The store data is driven on the result buses by the functional units.

    ______________________________________                                        RESLA0B 14:0!   Input from FU                                                 RESLA1B 14:0!   Input from FU                                                 RESLA2B 14:0!   Input from FU                                                 RESLA3B 14:0!   Input from FU ICLK10                                          ______________________________________                                    

Linear addresses from the functional units. The result buses are slow.The linear addresses need to come to the LSSEC faster so that it canperform two accesses at the end of the cycle. If needed, the bus widthcan be reduced to 14 bits--so that the cache indexing can be done. Therest of the bits can be grabbed from the result buses.

    ______________________________________                                        RSTAT0LS 2:0!   Input from FU                                                 RSTAT1LS 2:0!   Input from FU                                                 RSTAT2LS 2:0!   Input from FU                                                 RSTAT3LS 2:0!   Input from FU ICLK3                                           ______________________________________                                        00     Null                                                                   01     Address is driven by the functional unit on the RESLA bus              10     Data is driven on the result bus by the functional                     ______________________________________                                               unit                                                               

Bit 2=1 indicates that the linear address generated is for a loadinstruction. This is used by the LSSEC to forward linear addresses tothe data cache directly to the functional units.

    ______________________________________                                        RTAG0B 2:0!    Input from FU                                                  RTAG0B 2:0!    Input from FU                                                  RTAG0B 2:0!    Input from FU                                                  RTAG0B 2:0!    Input from FU ICLK3                                            ______________________________________                                    

This gives the ROB line number of the instruction that the FU isprocessing.

    ______________________________________                                        WRPTR 5:0!    Input from FIROB                                                                              ICLK5                                           DTBRKPT       Output to LSCTL ICLK5                                           DBMTCH        Output to LSCTL ICLK5                                           LSLMTCHK 1:0! Output to LSCTL ICLK5                                           LSRDAT 31:0!  Output to LDSTDAT                                                                             ICLK5                                           ______________________________________                                    

DCACHE

Overview

The data cache (henceforth referred to as dcache) on Processor 500 is a16 KB linearly addressed, 8-way set associative cache. In order tofacilitate single cycle dcache access, a way prediction scheme is used.This section discusses a 16 KB dcache.

Some of the other salient features of this dcache are 8-wayinterleaving, two concurrent accesses per cycle if they are not to thesame bank, random replacement policy and one cycle penalty for unalignedloads and unaligned stores.

Dcache Organization

The dcache on Processor 500 is a 16 KB linearly addressed cacheimplementing the MESI protocol. The line size is 32 bytes and the arraysare organized in a 8-way set associative structure with 8 banks. The 8banks allow two concurrent accesses per cycle as long as the twoaccesses are not to the same bank. Bits 4:2 of the two linear addressesfrom the Processor 500 core are used for bank selection and identifyingbank conflicts. Due to the interleaving, the data arrays are effectivelydual ported and do not need to be implemented as actual dual portedstructures. FIG. 52 shows a diagram of the dcache organization.

The dcache is partitioned into three functionally separate arrays. Theyare the tag array, data array and the way prediction array. The tagarrays are physically dual-ported. Since it is a 16 KB dcache with a 32byte line size, there are 64 tags per way. Bits 10:5 of the linearaddress are used to index the tag array with bits 31:11 used forcomparison to determine hit or miss.

The data arrays are organized as 8 set arrays in each of the 8 banks. Inorder to facilitate 8/16 bit accesses as well as unaligned accesses, theLSSEC will generate byte enables for each of the two accesses. Bits 10:5of the linear address are used to index the data arrays. The two portsthat allow two concurrent accesses per clock cycle will henceforth becalled Port A and Port B. The actions supported on Port A are: alignedloads/stores, unaligned loads/stores, reloads, SRB accesses, linereads(both snoop and non-snoop) and line invalidations. The actionssupported on Port B are aligned/unaligned loads, unaligned stores(sincea unaligned store uses both ports) and snoop invalidations.

Way prediction will allow the Processor 500 dcache to attain singlecycle access at our targeted clock speeds while using a set-associativecache array. Single cycle dcache access is feasible only when thepredicted way hits in the tag compare. If a hit occurs in any of theunpredicted ways then there is a one cycle penalty which would beequivalent to a replacement cache performance. However, unlike areplacement cache there will be no swap penalty. The way predictionarray entry is updated with the new predicted value. Bits 13:5 of thelinear address are used to index the way prediction array. Like the tagarray, the way prediction array is also dual ported. The details of theway prediction scheme are described below.

Tag Array

The tag array is organized into an 8-way set associative structure. Itis dual ported so as to allow two concurrent accesses per cycle and islaid out as one 64 rows×200 column array. This is optimal since thespeed target does not allow a contiguous array larger than 64 rows. Bits10:5 of the port linear address will be used to index the array. Thiswill be true for both port A and B accesses. A tag array entry per wayconsisting of 25 bits is as shown in FIG. 53.

A description of the various fields is as follows:

Tag is bits (31:11) of the linear address.

D is the dirty bit that indicates that the line has been previouslymodified. This information is used during a store by the LSSEC when theTLB is accessed to determine whether the corresponding dirty bit in thepage table entry is correctly set. If the dirty bit in the page tableentry is not set then an exception must occur to write the dirty bit inthe external page table entries so that the page gets written back toexternal memory.

U/S* is the user/supervisor bit that indicates the access privilege ofthe dcache line. If this bit is 0 then user level programs cannot accessthis dcache entry. The supervisor can access any line regardless of thisbit.

R/W* indicates the read/write privilege for user level programs.Supervisor level programs ignore this bit when the WP bit in CR0register is 0. If the WP bit is set to 1 then supervisor level programswill use the R/W* bit. The dcache does the protection checking and sendsan exception to the LSSEC if a violation occurs.

V is the linear valid bit. This bit is 0 on reset and is set to 1 when anew line gets loaded into the dcache. This bit is also reset to 0 oninvalidation.

The tag and the protection bits (D, U/S* and R/W*) are always accessedtogether. The V bit can be accessed independently for invalidations andresets.

Data Array

The data array is effectively dual ported due to banking. Each bank isphysically laid out as one 64 row×256 column array. This again isoptimal since the speed target for Processor 500 does not allow acontiguous array larger than 64 rows. During a clock cycle (regularload/store accesses), at most two banks can be accessed. The banks areselected by the bank select bits 4:2 of the port addresses. Bits 10:5 ofthe port linear address will be used to index each selected bank. Thedata array is byte addressable. Two sets of byte enables are generatedper clock corresponding to the two banks being accessed. The byteenables are generated using the operand size information as well as bits1:0 of the linear address. The byte enables are critical to doing storesin a single cycle for aligned accesses since stores are done as purewrites. Unaligned accesses and 8/16 bit accesses use byte enableinformation in the same fashion as well. All 8 banks are accessed onport A only during a reload or during a writeback. At this time (i.e.during reload or writeback) port B is unused.

Way Prediction Array

The way prediction array is a 512 rows×8 columns direct mapped cachethat is physically organized into eight 64×8 arrays. Each columncorresponds to one of the eight ways of the data and tag arrays as shownin FIG. 54. The way prediction array is implemented as a dual portedarray to allow two concurrent accesses per cycle. It is indexed by bits13:5 of the port A and B linear addresses. Note that unlike the data andtag array the index here is 9 bits wide. Therefore for each tag/dataarray index, there are 8 possible indexes in the way predict array(provided by extra index bits 13:11). It is noted here that greater themapping (i.e. more index bits for way prediction array), the moreaccurate the way prediction. This mapping was determined to be optimalbased on performance/implementation tradeoffs. Each of the eight arrayswill be addressed by bits 10:5 of the linear address. Bits 13:11 willselect the output of one of the eight arrays.

Aligned Loads

There are two cases to be considered here.

(a) 32 bit aligned loads and

(b) 8/16 bit aligned loads.

FIG. 56 shows a block diagram of the hardware to do aligned loads forboth these cases.

The dcache receives the linear address before the end of the 7th XICLK.The LSSEC also sends the index conflict and way conflict signals beforethe end of the 7th XICLK. At the beginning of the 8th XICLK, the dcachelatches these signals, does the row decode and also the bank selection.The index conflict and way conflict signals are used to fire only oneport of the dual ported tag and way prediction arrays. If a bankconflict occurs the LSSEC serializes the two accesses. Therefore, forthat clock cycle, only one dcache access will be issued on port A. TheLSSEC should also in the 8th XICLK send the shift code that is based onthe operand size and bits 1:0 of the port linear address and also thebyte enables. For the 32 bit load, the shift code is zero. The data fromthe predicted way will be driven on the LSRESn (n=0 or 1) and XLSRESnbuses differentially towards the end of the 8th XICLK. The appropriatefunctional unit latches this data at the beginning of the 9th XICLK anduses it. The dcache meanwhile computes the hit/miss indication for the 8ways and sends that indication to the functional unit and LSSEC. Ifthere was a hit in the predicted way, then there is single cycle dcacheaccess and the dcache can proceed with the next pending access. If therewas a miss in the predicted way and a hit in one of the unpredictedways, the LSSEC will cancel the next access that it had initiated onthat port and drive the previous tag again on the LSTAGn bus. Thefunctional unit will then cancel the operation and wait for data to bedriven to it again in the 9th XICLK. The data from one of theunpredicted ways is driven to the functional unit during the 9th XICLKand is latched at the beginning of the 10th XICLK. Therefore, there is aone cycle penalty when there is a miss in the predicted way and a hit inone of the unpredicted ways. Also, the way prediction array will beupdated with the new prediction in the 9th XICLK. FIG. 55 shows a timingdiagram of the above stated actions.

For the 8/16 bit loads that are contained within one doubleword, theflow is the same except that the shift logic that is present before thebus driver is utilized. This logic is present for the 32 bit loads also,but it is a shift by 0. This shift logic can be implemented usingtransmission gate muxes. For the 8 bit data, we can have either a shiftby 0,8,16 or 24. For 16 bit data, it is a shift by 0, 8 or 16. The shiftwill be determined by the operand size information and bits 1:0 of theport linear address. Both of these controls should be known at thebeginning of the 8th XICLK and the muxes can be setup before the data isavailable.

If there is a miss on all the ways, the LSSEC marks that access as amiss and proceeds with the next dcache access if one is pending. TheLSSEC also sends the address of the missed access to the CMASTER tocheck for an alias. If no aliasing has occurred, the LSSEC waits untilthe missed access is no longer speculative and then will initiate areload sequence.

Aligned Stores

Stores on Processor 500 are done as pure writes. The dcache on Processor500 supports byte write capability which allows pure writes. The byteenables used to do this are generated based on the operand size and bits1:0 of the port linear address. Processor 500 dcache supports singlecycle dcache access for stores if the store is to the predicted way.FIG. 57 is timing sequence of the actions that take place during thestore.

The sequence of actions until the dcache access are similar for both theload and store accesses. When the senseamps are turned on, the arraysare isolated from the senseamp to prevent the bit lines from beingpulled down. Thus, the senseamp provides an automatic latch structure tohold the read data. When the way prediction is available, the writestrobe for the predicted way is turned on thereby writing the store dataspeculatively into the predicted way. At the beginning of the 9th XICLK,the hit/miss indications are generated and protection violationsdetected. If there was a hit to the predicted way and no protectionviolation, then a single cycle dcache store access has been performed.If there is a miss in the predicted way but a hit in one of theunpredicted ways and no protection violations, then in the 9th XICLK thestore data is written into the way that hit and the predicted way isrestored with the data that was read out in the 8th XICLK. Also the wayprediction array is updated with the new prediction. If there is aprotection violation, then the predicted way is restored with the datathat was read out in the 8th XICLK. The protection violation informationis sent to LSSEC to generate an exception. The access is thus terminatedand the dcache can proceed with the next pending access on the followingclock cycle.

For 8/16 bit stores, the above actions still hold except that dependingon the byte enables that are active, only enabled bytes are updated withthe new store data.

Unaliaged Loads

Unaligned loads on Processor 500 require at least a two cycle dcacheaccess. FIG. 58 illustrates the sequence of actions for unaligned loadswhen both accesses hit in the predicted way.

In the 8th XICLK, the LSSEC will access the dcache with the currentaddress. During this time, the LSSEC also increments the address by 4 toset up for the second half of the unaligned access. The dcache latchesthe first half of the unaligned load at the beginning of the 9th XICLKand does the shift necessary to set up the data to drive on theappropriate bits of the LSRESn and XLSRESn buses. During the 9th XICLK,the dcache reads the second half of the unaligned load. The actionstaken are exactly the same as for a 8/16 bit aligned load. The twohalves of the data will be driven on the LSRESn and XLSRESn buses at theend of the 9th XICLK. Note that in this process, the byte enables areused to select the appropriate bytes from the two doublewords that havebeen read out.

There are three more cases that are considered:

(a) Unpredicted way hit in first access--Predicted way hit in secondaccess. FIG. 59 shows the timing diagram for handling this case. Thiscase occurs both when the unaligned load is contained within a line andwhen the load crosses a line boundary. As can be seen from FIG. 59, thiscase requires 3 clock cycles.

(b) Predicted way hit in first access--Unpredicted way hit in secondaccess. FIG. 60 shows the timing diagram for handling this case. Thiscase occurs when the unaligned load crosses a line boundary. This isbecause if the access were completely contained within the same line,there would not be a unpredicted way hit in the second access. This casealso requires 3 clock cycles.

(c) Unpredicted way hit in first access--Unpredicted way hit in secondaccess. This case will also occur when the unaligned load crosses theline boundary. This case will require 4 clock cycles to complete. FIG.61 shows the timing diagram for this case.

The cases covered so far only involved hits in the dcache. There are twocases of misses: Miss in first access and miss in second access. Whenthere is a miss during the first access, the LSSEC will mark that loadas a miss. The LSSEC will then process the miss as described below. Ifthere is a miss on the second half of the unaligned load, the first halfof the unaligned load that has been read out will be discarded. Thiscase would occur when the load crosses a line boundary. The LSSEC setsthe M2 bit in the LSSEC buffer and then processes the miss as describedbelow.

Unaligned Stores

Unaligned stores are executed in a slightly different fashion fromunaligned loads. For unaligned stores, the incremented address isgenerated in the 8th XICLK. In the 9th XICLK, port A is accessed withthe original address and port B with the incremented address. The byteenables for both the ports are used to write the appropriate bytes ofthe two doublewords. If there is a miss on any one of the ports then theold data that is read out on the other port is restored in the nextcycle. The LSSEC then processes the store miss as described below. FIG.62 shows a timing diagram when both store accesses hit in the predictedway.

The other two cases that are considered are:

(a) At least one of the two accesses misses in the predicted way buthits in an unpredicted way. FIG. 63 shows the timing diagram for thiscase.

(b) There is a miss in all the ways for at least one access. FIG. 64shows the timing diagram for each case.

Processing a Load Miss

On a load miss, the LSSEC sends the address on the DADDR bus to theCMASTER to check for aliasing. If it is a speculative request, thenLSSEC does not assert the ROB₋₋ REL signal. The CMASTER in this casechecks for an alias. If aliasing has occurred, the CMASTER sends theCM₋₋ DC₋₋ ALIAS signal back to LSSEC and dcache and also drive the wayto be replaced to the dcache on the CM₋₋ REPCOL bus and the protectioninformation on PAGE₋₋ PROT(2:0). The LSSEC, upon receiving the aliasingindication, drives the LSLINADO bus with the tag and index on thefollowing cycle and also assert the LSUPDATERD signal. This causes thenew tag and protection bits to be written in the selected way on thenext cycle. The valid bit for that way also set. See FIG. 65 for timingdiagram.

If there is a miss in the physical tags, the CM₋₋ MISS is asserted. TheLSSEC then waits until the dcache request that caused the miss is nolonger speculative and then again sends the address on DADDR bus. TheROB₋₋ REL signal is now asserted. The CMASTER latches the address andchecks for aliasing again. If no aliasing has occurred, it will initiatethe bus cycle and fetch the line in four packets of 64 bits each(external data bus=64 bits). The CMASTER can transfer each packet to thedcache on the DATB bus as soon as it is received. The dcache will latcheach packet into the appropriate location in a 256 bit buffer(BUF1). Thedcache on receiving the first packet asserts DCFSTPCK to the LSSEC. TheLSSEC, on the next cycle, sends the reload address(tag and index) todcache. After this point the LSSEC does not issue any more requests todcache. The CMASTER also selects the way in the dcache to be replacedand sends that selection to the dcache along with the last packet on theCM₋₋ REPCOL(2:0) bus. The last packet will be flagged by the CMASTER bythe assertion of CM2LS. The replacement policy is pseudo-random and isdetermined in the CMASTER. After all 4 packets have been received, thedcache asserts the DCBUSY signal and then initiates the line swap. Thedcache reads the line to be replaced into another 256 bit buffer(BUF2)and writes the line in BUF1 into that way. This swap can be done in oneclock cycle. Note that this is a store to all eight banks and hencethere is no port B access. The dcache will also update the tag and wayprediction arrays. The way into which the new line was put will be thenew way prediction for that index. See FIG. 66 for a timing diagram forreloads. If the line that was replaced was dirty, the CMASTER requeststhe dcache to send the line from BUF2. The dcache sends the line in fourpackets of 64 bits each on the DATB bus. The CMASTER then initiates anexternal write cycle to send each of these packets as they are received.

Processing a Store Miss

The Processor 500 dcache implements a no-write allocate policy for storemisses. A no write allocate policy will still require the CMASTER tocheck for aliasing using the same protocol described for load misses. Ifno aliasing occurred, the LSSEC initiates a memory write cycle throughthe CMASTER. The data is sent to the CMASTER from the LSSEC on the DATBbus. The dcache is not involved in this transaction.

Non-Cacheable Accesses

Non-cacheable accesses are detected in three ways: The first is when thePCD bit is set. The second is through the sampling of KEN# inactiveduring the same clock that the first BRDY# is sampled active. The thirdis when the CD and NW bits in the CR0 register are programmed as followsCD=1, NW=0. The actions when a miss occurs are the same for all thesethree cases.

For a miss (load or store), the LSSEC still sends the address to theCMASTER to check for aliasing. If aliasing has occurred then theprocedure for handling of non-cacheable accesses is the same as forcacheable accesses (see above). For a load miss, the LSSEC requests theCMASTER to initiate an external memory read cycle. The CMASTER reads therequested doubleword(or less if a byte or word is requested) and sendsit to the dcache over the DATB bus. This data is latched in the dcache.The dcache will inform the LSSEC that the data has been transferred. TheLSSEC then drives the tag for that load on the LSTAG0 bus and alsocommands the dcache to route/mux the data so as to drive the appropriatebits of the LSRES0 and XLSRES0 buses.

For a store miss (if no aliasing), the LSSEC initiates a memory writecycle through the CMASTER. The data is sent to the CMASTER from theLSSEC on the DATB bus. The dcache is not be involved in thistransaction.

Dcache Hooks to Support Snooping

The dcache is involved when an inquire cycle hits a modified line sincea writeback cycle is issued to update the modified line in externalmemory. The dcache is also involved during snoop invalidations.

Dcache/SRB Interface

The dcache interfaces to the SRB through one bi-directionalbus--SRBB(31:0) and two unidirectional signals SRB₋₋ VAL and DCTARVAL.The functions that can be requested by the SRB of the dcache includearray reads/writes, line invalidations and global invalidations.

An array read cycle is initiated by the SRB when it wishes to perform anarray read. The array to be accessed (e.g. dcache, icache . . . ) andthe action performed are specified in the first packet sent on the SRBBbus with the SRB₋₋ VAL signal in cycle N. The SRB will then send out thearray locator doubleword on the SRBB bus in cycle N+2. The dcache willlatch this array locator at end of cycle N+2 and read the appropriateword in cycle N+6. The dcache drives the requested data on the SRBB busand also assert the DCTARVAL signal during cycle N+7. Note that theSRB/dcache interface is slow and hence, the data read from the array canbe latched and driven out on a later clock cycle.

An array write cycle is initiated by the SRB when it wishes to write toan array. During cycle N, the SRB will specify the array to be accessedand the action to be performed. The SRB then sends out the array locatordoubleword on the SRBB bus in cycle N+2. The dcache latches this arraylocator at end of cycle N+2. During cycle N+4 the SRB sends the data tobe written into the dcache. The dcache writes this data in cycle N+6.The dcache will assert the DCTARVAL signal during cycle N+7 to indicatethe end of the write operation.

For invalidations, the SRB initiates the array invalidate bus cycle onthe SRBB bus in cycle N. The array locator is sent on the SRBB bus incycle N+2. The type of invalidation (line/global) is based on the OPfield in the array locator. During cycle N+6, the dcache executes theinvalidation requested(line/global). In cycle N+7, the dcache assertsthe DCTARVAL signal to indicate end of the requested operation.

Dcache Top Level I/O Signal List

DATB(63:0): Input/Output Connects to LSSEC/CMASTER

This is a bi-directional bus between the CMASTER, LSSEC and the dcache.This bus has the following uses:

(a) to send data to the dcache from the CMASTER during reloads.

(b) to send data to the CMASTER from the dcache during writebacks.

(c) to send data to CMASTER from the LSSEC for non-cacheable stores(bits 31:0 only)

SRBB(31:0): Input/Output Connects to SRB/LSSEC/CMASTER/ICACHE

This bi-directional bus is used by the SRB to move data to/from arraysand special registers, to send the array locator and to send the commandword.

SRB₋₋ VAL: Input/Output of SRB

This signal is asserted by the SRB whenever it sends the command word onthe SRBB bus to initiate an access.

LSLINAD0(31:2): Input. Connects to

    ______________________________________                                        LSSEC/FIROB/CMASTER     ICLK14                                                ______________________________________                                    

This bus carries the dcache port A linear address. In addition to havingthe port A linear address for loads/stores, this bus will also carry thelinear address when a reload from the CMASTER is ready to be done. i.e.the new line from CMASTER has been read into the 256 bit buffer(BUF1)that exists in the dcache and it is ready to be put in place of thereplaced line.

    ______________________________________                                        LSLINAD1(31:2): Input. Connects to                                                                     ICLK14                                               LSSEC/FIROB/CMASTER                                                           ______________________________________                                    

This bus carries the dcache port B linear address.

    ______________________________________                                        SBADAT(31:0): Input/Output of LSSEC                                                                     ICLK13                                              ______________________________________                                    

This bus drives the store data for port A. This is a static bus. Thisbus can be driven from the beginning of the clock thereby ensuring thatthe data has enough time to propagate to the dcache and CMASTER blocks.This bus is required because the LSRES0 bus cannot be used for thispurpose. The LSRES0 and XLSRES0 buses are differential buses that areevaluated by a senseamp on the falling edge of XICLK. Therefore the datawould be available only in the next cycle which would prevent singlecycle stores.

    ______________________________________                                        SBBDAT(31:0): Input/Output of LSSEC                                                                     ICLK13                                              ______________________________________                                    

This bus drives the store data for port B. This is a static bus. Thisbus can be driven from the beginning of the clock thereby ensuring thatthe data has enough time to propagate to the dcache and CMASTER blocks.This bus is required because the LSRES1 bus cannot be used for thispurpose. The LSRES1 and XLSRES1 buses are differential buses that areevaluated by a senseamp on the falling edge of XICLK. Therefore the datawould be available only in the next cycle which would prevent singlecycle stores.

    ______________________________________                                        LSPASHFT(1:0): Input/Output of LSSEC                                                                    ICLK14                                              ______________________________________                                    

This bus determines how much the data that is read from the dcache onport A should be shifted. This shift code is determined based on theoperand size as well as bits 1:0 of the linear address.

    ______________________________________                                        LSPASHFT(1:0)        Shift Value                                              ______________________________________                                        00                   shift by 0                                               01                   shift by 8                                               10                   shift by 16                                              11                   shift by 24                                              ______________________________________                                        LSPBSHFT(1:0): Input/Output of LSSEC                                                                    ICLK14                                              ______________________________________                                    

This bus determines how much the data that is read from the dcache onport B should be shifted. This shift code is determined based on theoperand size as well as bits 1:0 of the linear address.

    ______________________________________                                        LSPBSHFT(1:0)        Shift Value                                              ______________________________________                                        00                   shift by 0                                               01                   shift by 8                                               10                   shift by 16                                              11                   shift by 24                                              ______________________________________                                        LSINDXCT: Input/Output from LSSEC                                                                       ICLK14                                              ______________________________________                                    

This signal indicates an index conflict for the two dcache accesses. Anindex conflict is generated if bits 11:5 of the two linear addressesmatch. This signal will be used to read only one port of the dual portedtag arrays. As long as there is no bank conflict, this signal will notcause any serialization of dcache accesses.

    ______________________________________                                        LSWYPRCT: Input/Output from LSSEC                                                                       ICLK14                                              ______________________________________                                    

This signal indicates an index conflict for the way prediction array indcache. Note that the index for this array is bits 14:5 of the linearaddress. This signal will be used to fire only one port of the waypredict array.

    ______________________________________                                        CM2LS.sub.-- CPBK(3:0): Input/Output from CMASTER                             ______________________________________                                    

The CMASTER sends the information as to which 64 bit packet of the 32byte line is to be sent by the dcache on the DATB bus during awriteback. The decoding of this bus is as follows:

    ______________________________________                                        CM2LS.sub.-- CPBK(3:0)                                                                         Actions taken                                                ______________________________________                                        0001             bits 63:0 of writeback line                                                   driven on DATB(63:0)                                         0010             bits 127:64 of writeback line                                                 driven on DATB(63:0)                                         0100             bits 191:128 of writeback line                                                driven on DATB(63:0)                                         1000             bits 255:192 of writeback line                                                driven on DATB(63:0)                                         ______________________________________                                        RD.sub.-- SNP: Input                                                                             Output of CMASTER                                          ______________________________________                                    

The CMASTER sends this signal to the dcache to start driving bits 63:0of the line that has been read as part of a snoop writeback(initiated byCM₋₋ DC₋₋ SNP) on the DATB bus. On the next XICLK, the dcache will drivebits 127:64 of the line. The dcache will then drive bits 191:128 and255:192 on the two following clocks.

    ______________________________________                                        LS.sub.-- IDX(5:0): Input/Output of CMASTER                                   ______________________________________                                    

This is the index sent to the dcache from the CMASTER to be used forsnoop read, snoop invalidates, writebacks and non-snoop invalidations.

    ______________________________________                                        CM2DC(3:0): Input/Output from CMASTER                                         ______________________________________                                    

This bus is part of the reload sequence. It indicates which 64 bitpacket of the requested line is being sent to the dcache from theCMASTER on the DATB bus.

    ______________________________________                                        CM2DC(3:0)   Actions taken                                                    ______________________________________                                        0001         bits 63:0 of requested lin                                                    driven on DATB(63:0) by CMASTER                                  0010         bits 127:64 of requested line                                                 driven on DATB(63:0) by CMASTER                                  0100         bits 191:128 of requested line                                                driven on DATB(63:0) by CMASTER                                  1000         bits 255:192 of requested line                                                driven on DATB(63:0) by CMASTER                                  LSDCEXC: Input/Output of LSSEC                                                                         ICLK13                                               ______________________________________                                    

This signal informs the dcache that an exception has been detected bythe LSSEC during its protection checking and limit checking actions.This signal is used only during stores. When this signal goes activeduring a store, we restore the cache location that was written.

    ______________________________________                                        LDORST(1:0): Input/Output from LSSEC                                                                    ICLK12                                              ______________________________________                                    

LDORST(0) indicates whether the port A access when issued from the LSSECis a load or a store. LDORST(0)=0 indicates a load and LDORST(0)=1indicates a store. LDORST(1) does the same for port B.

    ______________________________________                                        PABYTEN(3:0): Input/Output from LSSEC                                                                   ICLK14                                              ______________________________________                                    

These are the byte enables for the dcache port A access. They will beused in unaligned accesses and 8/16 bit accesses.

    ______________________________________                                        PBBYTEN(3:0): Input/Output from LSSEC                                                                   ICLK14                                              ______________________________________                                    

These are the byte enables for the dcache port B access. They will beused in unaligned accesses and 8/16 bit accesses.

    ______________________________________                                        LSUNALGN(1:0): Input/Output of LSSEC                                                                    ICLK13                                              ______________________________________                                    

LSUNALGN(0) indicates that the load/store access to the dcache from portA is unaligned. LSUNALGN(l) indicates the same for port B. An unalignedload can be issued to either port A or port B. An unaligned store usesboth ports simultaneously.

    ______________________________________                                        BIU.sub.-- NC: Input/Output of CMASTER                                        ______________________________________                                    

When there is a miss in the dcache, this signal when active indicatesthat the external data access caused by the miss is non-cacheable.

    ______________________________________                                        CM.sub.-- REPCOL(2:0): Input/Output from CMASTER                              ______________________________________                                    

This bus selects the way to be replaced during an external reload cycle.Note that the pseudo-random replacement strategy will be implemented inthe CMASTER. It will also be used during writebacks and non-snoopinvalidations.

    ______________________________________                                        IRESET: Input                                                                 ______________________________________                                    

This is the global internal reset signal. All entries in the dcache mustbe invalidated on assertion of IRESET.

    ______________________________________                                        PAGE.sub.-- PROT(2:0): Input/Output of CMASTER                                ______________________________________                                    

These are the page protection bits(D, U/S*,R/W*) that are sent by theTLB on dcache misses. They are written into the tag array. In the caseof non-cacheable accesses, the PAGE₋₋ PROT bits should be used for pageprotection checks.

    ______________________________________                                        CM.sub.-- DC.sub.-- INV: Input/Output of CMASTER                              ______________________________________                                    

This signal is used to (non-snoop)invalidate a dcache line.

CM₋₋ DC₋₋ RD: Input/Output of CMASTER

This signal is used by the CMASTER to initiate a dcache read as part ofthe reload or writeback sequence.

CM₋₋ DC₋₋ SNP: Input/Output of CMASTER

Used by the CMASTER to initiate a snoop related read of the dcache.

CM₋₋ DC₋₋ SINV:Input/Output of CMASTER

Used by the CMASTER to do a snoop invalidation of a dcache line.

CM2LS₋₋ RST: Input/Output of CMASTER

Used to restart the CMASTER--dcache line transfer if the transfer washalted. For example, if BOFF# was asserted in the middle of thistransfer, Processor 500 would relinquish the external bus to another busmaster. When BOFF# is de-asserted, the bus cycle that was interruptedhas to be restarted. Therefore, the all four packets would betransferred regardless of how many had been transferred before theassertion of BOFF#.

CM2LS: Input/Output from CMASTER

This signal is a flag from the Cmaster to the dcache and the LSSEC thatthe last packet of a reload sequence is being sent.

CM2LS₋₋ CAN: Input/Output from CMASTER

This signal is used by the CMASTER handles a special case ofinvalidation. If during the reload sequence, we get a snoop invalidatefor the new line that is to be loaded into the dcache. This signal isused to reset the valid bit for the new line when it is loaded into thecache when the line swap occurs during the reload.

CR0WP: Input/Output from SRB

This is the write protect(WP) bit in the CR0 special register. This bitis used with the page protection bits to determine protectionviolations.

    ______________________________________                                        CURCPL(1:0): Input/Output of LSSEC                                                                      ICLK10                                              ______________________________________                                    

This is the Current Privilege level information. The dcache uses the D,R/W* and U/S* bits in conjunction with the CURCPL bits to determineprotection violations.

CM₋₋ DC₋₋ ALIAS: Input/Output of CMASTER

This signal from the CMASTER indicates to the dcache and LSSEC that analias was detected by the CMASTER. The CMASTER will also send the wayinformation on the CM₋₋ REPCOL bus along with this signal. The dcachewill use this to update the linear tag in the aliased way.

CR0PG: Input/Output from SRB

This is the paging enabled (PG) bit in the CR0 special register. Pagingis enabled when CR0PG=1.

    ______________________________________                                        LSUPDATERD: Input/Output from LSSEC                                                                     ICLK10                                              ______________________________________                                    

This signal is used to generate write signals to update the tag, statusand valid bits in the tag array when aliasing has occurred.

LS₋₋ SNP₋₋ COL(2:0): Input/Output from CMASTER

This bus selects the way in the dcache to be used for the writebackcycle that follows an inquire cycle. This writeback cycle occurs only ifthe inquire cycle hits a M-state line. It is also used to select the wayduring snoop invalidations.

    ______________________________________                                        LSRSLD(1:0) Input/Output from LSSEC                                                                     ICLK13                                              ______________________________________                                    

LSRSLD(0) indicates that there is a port A dcache access issued from theLSSEC. LSRSLD(1) does the same for port B.

    ______________________________________                                        LSCDFWD(1:0):Input/Output from LSSEC                                                                    ICLK11                                              ______________________________________                                    

LSFWD(0) indicates to the dcache not to drive the LSRES0/XLSRES0 buses.LSFWD(1) indicates to the dcache not to drive the LSRES1/XLSRES1 buses.

LSCPLISZERO:Input/Output from LSSEC

The protection checking in the dcache uses the current CPL bitsCURCPL(1:0). LSCPLISZERO being active will cause them to be overriddenwith zero.

LSCNMA(1:0): Input/Output from LSSEC

LSCNMA(0) when active, indicates to the dcache that it must cancel themisaligned load being executed on port A. This condition happens whenthe first half of the misaligned access is complete and the LSSEC findsout that it can forward the data from the store buffer. The LSSEC thenwill discard the data and will not drive the LSRES0 bus. LSCNMA(1)serves the same function for port B and the LSRES1 bus.

BSTRST:Input/Output from JTAG₋₋ BIST

This signal causes the address counter for the BIST logic in dcache toreset.

BSTRUN:Input/Output from JTAG₋₋ BIST

This signal causes BIST to start.

BSTRD:Input/Output from JTAG₋₋ BIST

This signal is asserted to read the dcache arrays and do the compare inthe TAEC registers.

BSTWR:Input/Output from JTAG₋₋ BIST

This signal is asserted to write the dcache arrays when in BIST mode.

BSTINCR:Input/Output from JTAG₋₋ BIST

This signal is asserted to request an increment of the BIST addresscounter.

BSTDIN:Input/Output from JTAG₋₋ BIST

This is the data to be loaded into the TAEC input registers prior tobeing written into the arrays.

FLUSHON: Input--Connects to JTAG₋₋ BIST

This signal enables the shifting of the data on the BSTDIN input intothe TAEC input registers.

UPDOWN:Input--Connects to JTAG₋₋ BIST

This signal control whether the BIST address counter will counter up ordown. UPDOWN=1 is up and UPDOWN=0 is down.

BSTSHF1:Input--Connects to JTAG₋₋ BIST

This is the signal that clocks the master latch in the TAEC registers.

BSTSHF2:Input--Connects to JTAG₋₋ BIST

This is the signal that clocks the slave latch in the TAEC registers.

BSTFALSE:Input--Connects to JTAG₋₋ BIST

BSTFALSE when active will cause the test pattern to be inverted.

PORTSEL:Input--Connects to JTAG₋₋ BIST

This signal when active selects the second port of the tag and waypredict arrays.

    ______________________________________                                        DCTARVAL: Output - Connects to SRB                                                                      ICLK8                                               ______________________________________                                    

This signal is asserted by the dcache to indicate the end of therequested SRB operation. If the requested operation was a read, then thedcache will also drive the SRBB bus with the read data.

LSRES0(31:0): Output. Connects to LSSEC/FNCU/FIROB. ICLK14

This is the true portion of the LSSEC/dcache port A access result bus.On Processor 500, driving buses is a major task and it has beendetermined that for speed reasons this bus in conjunction withXLSRES0(31:0) will be a differential bus. Data is output from dcache onthis bus during loads.

XLSRES0(31:0): Output. Connects to LSSEC/FNCU/FIROB. ICLK14

This is the false portion of the LSSEC/dcache port A access result bus.As stated above, the XLSRES0 bus is part of a differential bus.

LSRES1(31:0): Output. Connects to LSSEC/FNCU/FIROB. ICLK14

This is the true portion of the LSSEC/dcache port B access result bus.On Processor 500, driving buses is a major task and it has beendetermined that for speed reasons this bus in conjunction withXLSRES1(31:0) will be a differential bus. Data is output from dcache onthis bus during loads.

XLSRES1(31:0): Output. Connects to LSSEC/FNCU/FIROB. ICLK14

This is the false portion of the LSSEC/dcache port B access result bus.As stated above, the XLSRES1 bus is part of a differential bus.

    ______________________________________                                        DCLSPROK(1:0): Output - Connects to LSSEC                                                                ICLK5                                              ______________________________________                                    

Indicates a protection violation during a dcache access. DCLSPROK(0)corresponds to port A and DCLSPROK(1) corresponds to port B.

    ______________________________________                                        DCLSD(1:0): Output - Connects to LSSEC                                                                  ICLK5                                               ______________________________________                                    

DCLSD(0) and DCLSD(1) are the dirty bits corresponding to port A andport B respectively. They are used by the LSSEC during stores todetermine if the corresponding dirty bit in the page table entry in theTLB is correctly set. If the store is released and we find that the pagetable entry thinks it is clean, then an exception must occur to writethe dirty bit into the external page table entries so as to insure thatthis page gets written back to external memory.

DCFINRLD: Output--Connects to LSSEC/CMASTER ICLK13

This signal indicates to the LSSEC and CMASTER that the reload has beencompleted. The LSSEC may now issue load/store requests to the dcache.The CMASTER on receiving this signal may request a writeback cycle ifthe replaced line was dirty.

    ______________________________________                                        DCDATRDY:Output - Connects to LSSEC                                                                     ICLK13                                              ______________________________________                                    

This signal informs the LSSEC that the data for a non-cacheable load isavailable to be driven on the LSRES0 bus. The LSSEC must then on thenext clock, drive the tag on the LSTAG0 bus when the dcache drives thedata on the LSRES0 bus.

    ______________________________________                                        DCFSTPCK: Output - Connects to LSSEC                                                                    ICLK11                                              ______________________________________                                    

This signal informs the LSSEC that the first packet for a reload hasarrived at the dcache. The LSSEC on the next cycle will cancel anyaccess it had in progress and send the reload address(tag and index) onthe LSLINAD0 bus.

    ______________________________________                                        DCPAPRHIT: Output - Connects to LSSEC/FNCU                                                               ICLK3                                              ______________________________________                                    

This signal indicates to the functional units and the LSSEC that therewas a hit on the port A predicted way. Single cycle dcache access isachieved on port A when this signal is active.

    ______________________________________                                        DCPBPRHIT: Output - Connects to LSSEC/FNCU                                                               ICLK3                                              ______________________________________                                    

This signal indicates to the functional units and the LSSEC that therewas a hit on the port B predicted way. Single cycle dcache access isachieved on port B when this signal is active.

    ______________________________________                                        DCUNPAHIT: Output - Connects to LSSEC/FNCU                                                               ICLK3                                              ______________________________________                                    

This signal indicates to the functional units and the LSSEC that therewas a hit in one of the port A unpredicted ways. Two cycle dcache accessis achieved on port A when this signal is active.

    ______________________________________                                        DCUNPBHIT: Output - Connects to LSSEC/FNCU                                                               ICLK3                                              ______________________________________                                    

This signal indicates to the functional units and the LSSEC that therewas a hit in one of the port B unpredicted ways. Two cycle dcache accessis achieved on port B when this signal is active.

    ______________________________________                                        DCBUSY: Output - Connects to LSSEC                                                                      ICKL12                                              ______________________________________                                    

This signal is asserted to inform the LSSEC that the dcache is busydoing either a snoop read, snoop invalidation, non-snoop invalidation,reload or a cache line read that is done prior to a reload. The LSSECmust not send any dcache accesses as long as this signal is active.

    ______________________________________                                        DCCNCLA:Output - Connects to LSSEC                                                                      ICLK7                                               ______________________________________                                    

This signal indicates to the LSSEC that it must cancel the port A accessand reissue it on the next clock. This signal will be activated when theport B access hits in the unpredicted way in cycle N. During cycle N,port A has hit in the predicted way and achieved single cycle access.Now in cycle N+1, the LSSEC issues another access to port A. Now ifthere is a bank conflict between the new access on port A and the oldaccess on port B, then we want to cancel the port A access. This signalwill achieve this purpose.

    ______________________________________                                        DCCNCLB:Output - Connects to LSSEC                                                                      ICLK7                                               ______________________________________                                    

This signal indicates to the LSSEC that it must cancel the port B accessand reissue it on the next clock. This signal will be activated when theport A access hits in the unpredicted way in cycle N. During cycle N,port B has hit in the predicted way and achieved single cycle access.Now in cycle N+1, the LSSEC issues another access to port B. Now ifthere is a bank conflict between the new access on port B and the oldaccess on port A, then we want to cancel the port B access. This signalwill achieve this purpose.

BSTDDOUT:Output--Connects to JTAG₋₋ BIST

This signal is the output of the result register path of the TAEC chainfor the data array.

BSTDTOUT:Output--Connects to JTAG₋₋ BIST

This signal is the output of the result register path of the TAEC chainfor the tag and way prediction arrays.

MAXADDR:Output--Connects to JTAG₋₋ BIST

This signal indicates that the address counter for the arrays hasreached its maximum count.

Dcache Hierarchy and Internal Signal Definition

The dcache at the first level of hierarchy is partitioned into twoblocks: DCARRAYS and DCCTL. DCCTL will be the synthesized control blockand DCARRAYS will have all the custom logic. DCARRAY is furtherpartitioned into four blocks: DCTSVARRAY which is the tag, protection,valid bit array, DCDATARRAY which is the data array, DCWYPRARRAY whichis the way prediction array and DCDCBUS which will provide a cleaninterface to the arrays from the LSSEC and CMASTER. The internal signalsdown 2 levels of hierarchy will be described in the following foursections. For each block, only output signals will be described sincethe outputs of one block will be inputs of another and repetition can beavoided. Also signals that are top level I/O for the dcache will alsonot be described here. See above for their description.

DCCTL Signal List

DCDATWR(7:0): Output--Connects to DCDATARRAY ICLK9

This bus is the write strobe for each bank of the data array.

    ______________________________________                                        DCDATARRD(7:0): Output - Connects to                                                                    ICLK6                                               DCDATARRAY                                                                    ______________________________________                                    

This bus is the evaluate strobe for each bank of the data array. Thissignal is asserted for any access of the data array (read or write).

SRBVALP2: Output--Connects to DCDCBUS ICLK3

This signal is asserted two clocks after the SRB₋₋ VAL signal isreceived if the dcache is the target of the SRB access. SRBVALP2 is thenused to latch the array locator.

SRBVALP4: Output--Connects to all array blocks ICLK3

This signal is asserted four clocks after the SRB₋₋ VAL signal isreceived if the dcache is the target of the SRB access and the requestedoperation is a write. SRBVALP4 is then used to latch the write data.

    ______________________________________                                        SRBDCACC: Output       ICLK5                                                  ______________________________________                                    

This signal is a decode of the SRB control word(CW(4:0)=14h) that issent out in the first packet. It indicates that the dcache is the targetof the dcache access.

SRBWYRD: Output--Connects to DCDCBUS ICLK4

This signal indicates that the requested operation is a read of the waypredict array.

    ______________________________________                                        DCTSWR: Output - Connects to DCTSVARRAY/DCDCBUS                                                           ICLK3                                             ______________________________________                                    

This is the write strobe for the tag and protection bits in the tagarray. This signal is asserted during a reload or an SRB access orduring an update when an alias has been detected by the CMASTER. Writeto the tag and protection bits can occur only from port A.

    ______________________________________                                        DCPAVALWR:                 ICLK3                                              Output - Connects to DCDCBUS/DCTSVARRAY                                       ______________________________________                                    

This is the write signal corresponding to port A for the valid bit inthe tag array. This signal is asserted for either reloads, lineinvalidations, SRB access or updates when an alias is detected byCMASTER.

    ______________________________________                                        DCPBVALWR:                 ICLK3                                              Output - Connects to DCDCBUS/DCTSVARRAY                                       ______________________________________                                    

This is the write signal corresponding to port B for the valid bit inthe tag array. This signal is asserted for snoop invalidations.

    ______________________________________                                        DCPAVALIN: Output - Connects to DCTSVARRAY                                                                ICLK3                                             ______________________________________                                    

This is the bit corresponding to port A that is to be written into thevalid bit in the tag array. This bit will be 1 for reloads and updateswhen an alias has been detected. This bit will be 0 for lineinvalidations (through CMASTER and SRB).

    ______________________________________                                        DCLTRLDAD:                 ICLK3                                              Output - Connects to DCDCBUS/DCTSVARRAY                                       ______________________________________                                    

This signal when active indicates the presence of the reload/aliasedaddress(tag and index) on the LSLINADO bus. The DCDCBUS block will latchthe index and the DCTSVARRAY will latch the tag.

    ______________________________________                                        DCUSERLOAD:Output - Connects to DCDCBUS                                                                  ICLK1                                              ______________________________________                                    

This signal chooses the previously latched reload address for the nextdcache access on port A. The index and the column to be written(PAWRITECOL) will be generated in the DCDCBUS block. No port B accesswill occur.

    ______________________________________                                        DCENRLK:Output - Connects to DCTSVARRAY                                                                  ICLK2                                              ______________________________________                                    

This signal goes active when the address and data are ready to do areload.

    ______________________________________                                        DCENNC:Output - Connects to DCTSVARRAY                                                                   ICLK5                                              ______________________________________                                    

This signal goes active when the access on port A is a non-cacheableload.

    ______________________________________                                        DCALLBNKS:Output - Connects to DCDATARRAY                                                                 ICLK2                                             ______________________________________                                    

This signal is used to select all the banks of the data array in orderto do either a writeback or a reload. The port A index will be used forall the banks.

    ______________________________________                                        DCPABKEN:Output - Connects to DCDATARRAY                                                                 ICLK2                                              ______________________________________                                    

This signal enables the choosing of the appropriate bank for the port Aaccess. It is generated when either LSRSLD(0)=l or there is an SRBaccess.

    ______________________________________                                        SELPAIDX: Output - Connects to DCDCBUS                                                                   ICLK2                                              ______________________________________                                    

This signal selects the LS₋₋ IDX bus as the index for the port A dcacheaccess. This signal is active when a writeback (snoop and non-snoop) orinvalidation is being processed by the dcache.

    ______________________________________                                        DCSNPLIN:Output - Connects to DCDATARRAY                                                                 ICLK9                                              ______________________________________                                    

This signal when active will latch the line that is read out of the dataarray on a snoop read into a 32 byte buffer. The CMASTER will thenassert RD₋₋ SNP to indicate to the dcache that it may start driving theDATB bus with 64 bit packets of the data in this buffer.

    ______________________________________                                        SRBWYWR:Output - Connects to DCDBCBUS                                                                    ICLK5                                              ______________________________________                                    

This signal indicates that the requested operation is a write of the waypredict array.

    ______________________________________                                        SRBGLINV:Output - Connects to DCTSVARRAY                                                                 ICLK7                                              ______________________________________                                    

This signal will cause a global invalidation of the tag array.

    ______________________________________                                        SRBTAGRD:Output - Connects to DCDCBUS                                                                    ICLK5                                              ______________________________________                                    

This signal indicates that the requested operation is a read of thedcache tag array.

    ______________________________________                                        DRSRBDAT:Output - Connect to                                                                              ICLK3                                             DCTSVARRAY/DCWYPRARRAY/DCDATARRAY                                             ______________________________________                                    

This signal when active will drive the data from the chosen dcache arrayon the SRBB bus to the SRB.

DCDCBUS Signal List

    ______________________________________                                        DCPATSVRD: Output - Connects to DCTSVARRAY                                                                ICLK5                                             ______________________________________                                    

This is the port A evaluate signal for the tag array. This signal isasserted for any port A access of the tag array (read or write).

    ______________________________________                                        DCPBTSVRD: Output - Connects to DCTSVARRAY                                                                ICLK5                                             ______________________________________                                    

This is the port B evaluate signal for the tag array. This signal isasserted for any port B access of the tag array (read or write).

    ______________________________________                                        DCPAINDX(5:0): Output - Connects to                                                                     ICLK3                                               DCTSVARRAY/DCDATARRAY                                                         ______________________________________                                    

This is the index for the port A access to both the tag and data arrays.The index corresponds to either bits 10:5 of LSLINAD0 or LS₋₋ IDX(5:0).

    ______________________________________                                        DCPBINDX(5:0): Output - Connects to                                                                     ICLK3                                               DCTSAARRAY/DCDATARRAY                                                         ______________________________________                                    

This is the index for the port B access to both the tag and data arrays.The index corresponds to either bits 10:5 of LSLINAD1 or the LS₋₋IDX(5:0) or the bits of the SRBB bus corresponding to index.

    ______________________________________                                        WYAINDX(8:0): Output - Connects to DCWYPRARRAY                                                            ICLK3                                             ______________________________________                                    

This is the index for the way prediction array, port A access.Corresponds to bits 13:5 of the LSLINADO bus.

    ______________________________________                                        WYBINDX(8:0): Output - Connects to DCWYPRARRAY                                                            ICLK3                                             ______________________________________                                    

This is the index for the way prediction array, port B access.Corresponds to bits 13:5 of the LSLINAD1 bus.

    ______________________________________                                        WAYRDA: Output - Connects to DCWYPRARRAY                                                                  ICLK5                                             ______________________________________                                    

This is the port A evaluate signal for the way predict array. Thissignal is asserted for any port A access of the way predict array (reador write).

    ______________________________________                                        WAYRDB: Ouput - Connects to DCWYPRARRAY                                                                  ICLK5                                              ______________________________________                                    

This is the port B evaluate signal for the way predict array. Thissignal is asserted for any port B access of the way predict array (reador write).

    ______________________________________                                        WAYWRA: Output - Connects to DCWYPRARRAY                                                                 ICLK9                                              ______________________________________                                    

This is the port A write signal for the way predict array. This signalis asserted when updating the array when a misprediction occurs or whena reload occurs.

    ______________________________________                                        WAYWRB: Output - Connects to DCWYPRARRAY                                                                 ICLK9                                              ______________________________________                                    

This is the port B evaluate signal for the way predict array. Thissignal is asserted when updating the array when a misprediction occurs.

    ______________________________________                                        PABNKSEL(7:0): Output - Connects to DCDATARRAY                                                            ICLK5                                             ______________________________________                                    

This bus selects the bank in the data array for the port A access.

    ______________________________________                                        PBBNKSEL(7:0): Output - Connects to DCDATARRAY                                                            ICLK5                                             ______________________________________                                    

This bus selects the bank in the data array for the port B access.

    ______________________________________                                        DCPABNKSEL(2:0): Output - Connects to DCCTL                                                              ICLK3                                              ______________________________________                                    

This bus is the encoded version of PABNKSEL bus. It selects the bank inthe data array for the port A access.

    ______________________________________                                        DCPBBNKSEL(2:0): Output - Connects to DCCTL                                                              ICLK3                                              ______________________________________                                    

This bus is the encoded version of PBBNKSEL bus. It selects the bank inthe data array for the port B access.

    ______________________________________                                        DCWYXFER: Output - Connects to DCWYPRARRAY                                                                ICLK7                                             ______________________________________                                    

This signal is used for a very specific case. Consider the case asfollows: There are two load accesses to the dcache on cycle I. The portA access hits in the predicted way while the port B access hits inunpredicted way. Now in cycle I+1, there can be a new access on port Awhile the data from unpredicted way will be driven out on port B. Also,the way prediction array will be updated. Now if there is a way conflictbetween the new port A access and the old port B access, the way predictarray need not be read since the port B update to way predict array isthe prediction for port A. DCWYXFER will facilitate transfer of thisupdate from one port to the other port in such a case. The direction ofthe transfer can be determined by looking at the HIT signals for bothports(DCPAPRHIT,DCPBPRHIT,DCUNPAHIT,DCUNPBHIT).

PAWRITECOL(2:0):Output--Connects to DCTSVARRAY/DCDATARRAY ICLK4

This bus selects the way to be used for reloads, updates to fix aliases,SRB accesses and invalidations.

PBWRITECOL(2:0):Output--Connects to DCTSVARRAY/DCDATARRAY ICLK4

This bus selects the way to be used for snoop invalidations.

    ______________________________________                                        ARRTYP(2:0): Output - Connects to DCCTL                                                                  ICLK4                                              ______________________________________                                    

This bus carries the type information of the array locator word. ARRTYPspecifies the array in the dcache that the SRB wishes to access.

    ______________________________________                                        ARROP(1:0):Output - Connects to DCCTL                                                                   ICLK4                                               ______________________________________                                    

This bus carries the OP field of the array locator word. ARROP specifiesthe type of invalidation(line/global) to be done if the SRB requestedaction was a dcache invalidation.

DCTSVARRAY Signal List

PATAGCOM: Output--Connects to DCDATARRAY/DCWYPRARRAY/DCCTL ICLK14

This bus indicates the result of the tag compare for all 8 ways for portA. This bus will therefore provide the update to the way predict arrayif a misprediction occurs on port A.

PBTAGCOM: Output--Connects to DCDATARRAY/DCWYPRARRAY/DCCTL ICLK14

This bus indicates the result of the tag compare for all 8 ways for portB. This bus will therefore provide the update to the way predict arrayif a misprediction occurs on port B.

Turning now to FIG. 67, a computer system 1000 including microprocessor200, an interrupt controller 1004, a plurality of input/output (I/O)devices 1006A-1006N (referred to collectively as I/O devices 1006), abus bridge 1008, a main memory 1010, and a cache 1012 is shown.Microprocessor 200, interrupt controller 1004, bus bridge 1008, mainmemory 1010, and cache 1012 are coupled to a CPU bus 1014 which effectshigh speed, high bandwidth communication between the connected devices.Lower speed, lower bandwidth communication is provided for I/O devices1006 via input/output bus 1016. I/O devices 1006 are coupled tointerrupt controller 1004 via an interrupt bus 1018.

Interrupt bus 1018 includes at least one interrupt conductor per I/Odevice 1006A-1006N. An interrupt request signal conveyed on theassociated interrupt conductor is indicative of a request for interruptof microprocessor 200 by the associated I/O device 1006A-1006N.Interrupt controller 1004 is configured to receive the interrupt requestsignals and to convey an interrupt signal to microprocessor 200 upon CPUbus 1014 in response to the interrupt request signals. In oneembodiment, interrupt controller 1004 is an 8259A interrupt controllerused in IBM compatible personal computer systems. In another embodiment,interrupt controller 1004 includes multiple interrupt controllerssimilar to the 8259A cascaded to allow a larger number of I/O devices1006 to be connected. Interrupt controller 1004 is programmable, and maybe enabled to assert the interrupt signal individually for eachinterrupt request signal. In other words, interrupt controller may beenabled to assert the interrupt signal if the interrupt request signalfrom I/O device 1006A is asserted but not if the interrupt requestsignal from I/O device 1006B is asserted (or vice-versa). In anotherembodiment, interrupt controller 1004 may be programmed with a priorityscheme for the interrupt request signals. The priority scheme is devisedsuch that, if multiple interrupt request lines are assertedconcurrently: the highest priority interrupt request is service first;followed by the next highest priority interrupt request; etc.

Interrupt controller 1004 provides an interrupt vector which identifiesthe location of the interrupt service routine associated with theinterrupting I/O device 1006A-1006N. The interrupt vector is transferredacross CPU bus 1014 after microprocessor 200 acknowledges receipt of theinterrupt signal. In one embodiment, microprocessor 200 issues aninterrupt acknowledge transaction on CPU bus 1014 when acknowledging theinterrupt signal, and then issues a second interrupt acknowledgetransaction to collect the interrupt vector. Other transactions upon CPUbus 1014 are used to transfer data among devices coupled to CPU bus1014. An exemplary bus implementing such an interrupt acknowledgetransaction may be found within the publication entitled: "16-bit and32-bit Microprocessors Architecture, Software and InterfacingTechniques" by Singh, et al, Prentice Hall, Englewood Cliffs, N.J.,1991. This publication is incorporated herein by reference in itsentirety.

Bus bridge 1008 is provided to assist in communications (other thaninterrupt requests) between I/O devices 1006 and devices coupled to CPUbus 1014. I/O devices 1006 typically require longer bus clock cyclesthan microprocessor 200 and other devices coupled to CPU bus 1014.Therefore, bus bridge 1008 provides a buffer between CPU bus 1014 andinput/output bus 1016. Additionally, bus bridge 1008 translatestransactions from one bus protocol to another. In one embodiment,input/output bus 1016 is an Enhanced Industry Standard Architecture(EISA) bus and bus bridge 1008 translates from the CPU bus protocol tothe EISA bus protocol. In another embodiment, input/output bus 1016 is aPeripheral Component Interconnect (PCI) bus and bus bridge 1008translates from the CPU bus protocol to the PCI bus protocol. It isnoted that many variations of CPU bus protocols exist. Microprocessor200 may employ any suitable bus protocol.

I/O devices 1006 provide an interface between computer system 1000 andother devices external to the computer system. Exemplary I/O devicesinclude a modem, a serial or parallel port, a sound card, etc. I/Odevices 1006 may also be referred to as peripheral devices. Certain I/Odevices 1006 may also be used to enhance the performance of computersystem 1000 by performing certain tasks such that microprocessor 200 mayperform other tasks. Exemplary devices include hardware accelerators.Main memory 1010 stores data and instructions for use by microprocessor200. In one embodiment, main memory 1010 includes one or more dynamicrandom access memory (DRAM) cells and a DRAM memory controller. It isunderstood that main memory 1010 may be configured as multi-banked orinterleaved memory. Cache 1012 is a small, high speed memory fortemporarily storing data and instructions from main memory 1010. Ifmicroprocessor 200 requests an address from main memory 1010 and cache1012 is storing data associated with the requested address, then cache1012 provides the data. Cache 1012 is an optional component, andcomputer system 1000 is fully functional without it. However, cache 1012may improve the performance of computer system 1000. It is noted thatcache 1012 may also be inserted between microprocessor 200 and CPU bus1014 in other embodiments. Although FIG. 67 shows a computer system 1000including one microprocessor 200, computer system 1000 may includemultiple microprocessors similar to microprocessor 200 coupled to CPUbus 1014.

It is noted that a superscalar microprocessor in accordance with theforegoing may further employ the latching structures as disclosed withinthe copending, commonly assigned patent application entitled"Conditional Latching Mechanism and Pipelined Microprocessor Employingthe Same", Ser. No. 08/400,608 filed Mar. 8, 1995, by Pflum et al. Thedisclosure of this patent application is incorporated herein byreference in its entirety.

It is further noted that aspects regarding array circuitry may be foundin the co-pending, commonly assigned patent application entitled "HighPerformance Ram Array Circuit Employing Self-Time Clock Generator forEnabling Array Access", Ser. No. 08/473,103 filed Jun. 7, 1995 by Tran,now U.S. Pat. No. 5,619,464. The disclosure of this patent applicationis incorporated herein by reference in its entirety.

It is additionally noted that other aspects regarding superscalarmicroprocessors may be found in the co-pending, commonly assigned patentapplication entitled "Superscalar Microprocessor Including a HighPerformance Instruction Alignment Unit", Ser. No. 08/377,843, filed Jan.25, 1995 by Witt, et al. The disclosure of this patent application isincorporated herein by reference in its entirety.

In accordance with the above disclosure, a way prediction structure hasbeen described which enables access to an associative cache with anaccess time which is characteristic of a direct-mapped cache. Such acache and way prediction structure may be advantageously incorporatedinto a high frequency superscalar microprocessor. Performance may beincreased due to the better hit rates available with an associativecache and due to the shorter clock cycle achievable with the wayprediction structure.

Numerous variations and modifications will become apparent to thoseskilled in the art once the above disclosure is fully appreciated. It isintended that the following claims be interpreted to embrace all suchvariations and modifications.

What is claimed is:
 1. A superscalar microprocessor comprising a datacache including:a way prediction array including a plurality of storagelocations wherein each one of said plurality of storage locations isconfigured to store a way value, and wherein said way prediction arrayis configured to receive at least one request address; a way predictioncontrol unit coupled to said way prediction array wherein said wayprediction control unit is configured to update said way predictionarray; a second plurality of storage locations configured to store databytes and to receive said request address, wherein said second pluralityof storage locations is coupled to said way prediction array, andwherein one of said second plurality of storage locations is selected bysaid way value to convey data bytes upon an output bus; a tag comparisonblock, and wherein said tag comparison block is configured to receivesaid request address and said way prediction during a first clock cycle,and wherein said tag comparison block is coupled to said way predictionarray and to said way prediction control unit, and wherein said tagcomparison block is configured to convey a plurality of signalsindicative of the correctness of said way prediction, and wherein saidtag comparison block is configured to compare said request address tosaid plurality of tag addresses and to produce a hit unpredicted waysignal in response thereto, and wherein said hit unpredicted way signalindicates, when asserted, that said way prediction is incorrect and thatsaid request address hit in a way other than a particular way indicatedby said way prediction, and a reservation station coupled to receivesaid hit unpredicted way signal from said tag comparison block, whereinsaid reservation station is configured to invalidate a first datumforwarded by said cache in response to said way prediction if said hitunpredicted way signal is asserted.
 2. The superscalar microprocessor asrecited in claim 1 wherein said way prediction array is a linear array.3. The superscalar microprocessor as recited in claim 1 wherein saidsecond plurality of storage locations is an associative array.
 4. Thesuperscalar microprocessor as recited in claim 1 wherein said wayprediction control unit is configured to update said way predictionarray during a clock cycle in which said way prediction is found to beincorrect.
 5. The superscalar microprocessor as recited in claim 1wherein said data cache further includes a third plurality of storagelocations, and wherein said tag comparison block is adapted to receive aplurality of tag addresses from said third plurality of storagelocations.
 6. The superscalar microprocessor as recited in claim 5further comprising a load/store unit coupled to said data cache whereinsaid load/store unit is configured to execute load and store memoryaccesses.
 7. The superscalar microprocessor as recited in claim 6wherein said load/store unit is configured to access said data cachewith said request address during a second clock cycle.
 8. Thesuperscalar microprocessor as recited in claim 6 further comprising aplurality of functional units coupled to said load/store unit, whereinsaid plurality of functional units are configured to calculate addressesfor said load/store unit such that said load/store unit may perform saidload and store memory accesses.
 9. The superscalar microprocessor asrecited in claim 8 wherein said plurality of functional units arefurther configured to execute decoded instructions.
 10. The superscalarmicroprocessor as recited in claim 9 further comprising a plurality ofreservation stations configured to store said decoded instructions priorto execution by said plurality of functional units, wherein each of saidplurality of reservation stations is coupled to a respective one of saidplurality of functional units.
 11. The superscalar microprocessor asrecited in claim 10 further comprising a plurality of decode unitsconfigured to decode instructions into said decoded instructions,wherein each of said plurality of decode units is coupled to arespective one of said plurality of reservation stations.
 12. Thesuperscalar microprocessor as recited in claim 11 further comprising:aninstruction cache for storing said instructions; and an instructionalignment unit coupled between said instruction cache and said pluralityof decode units, wherein said instruction alignment unit is configuredto align said instructions from said instruction cache to said pluralityof decode units.
 13. The superscalar microprocessor as recited in claim12 further comprising a reorder buffer coupled to said plurality ofdecode units and to said plurality of functional units, wherein saidreorder buffer is configured to store speculative register states. 14.The superscalar microprocessor as recited in claim 13 further comprisinga register file for storing non-speculative register states, whereinsaid register file is coupled to said reorder buffer, and wherein saidreorder buffer is configured to store said speculative register statesto said register file in program order.
 15. The superscalarmicroprocessor as recited in claim 1 wherein said tag comparison blockis configured to produce a hit predicted way signal in response tocomparing said plurality of tag addresses, and wherein said hitpredicted way signal indicates, when asserted, that said way predictionis correct, and wherein said reservation station is configured todiscard an instruction which receives said first datum as an operand ifsaid hit predicted way signal is asserted and said instruction isexecuted upon receipt of said first datum.
 16. The superscalarmicroprocessor as recited in claim 1 wherein said comparison block isconfigured to assert a miss signal in response to comparing saidplurality of tag addresses to said request address if none of saidplurality of tag addresses match said request address, and wherein saidreservation station is configured to invalidate said first datum if saidmiss signal is asserted.
 17. A superscalar microprocessor comprising adata cache including:a way prediction array including a plurality ofstorage locations wherein each one of said plurality of storagelocations is configured to store a way value and wherein said wayprediction array is configured to concurrently receive a fat requestaddress and a second request address, and wherein said way predictionarray is configured to concurrently convey a first way prediction inresponse to said first request address and a second way prediction inresponse to said second request address; a way prediction control unitcoupled to said way prediction array, wherein said way predictioncontrol unit is configured to update said way prediction array; a tagcomparison block, wherein said tag comparison block is coupled toreceive said first way prediction, said second way prediction, saidfirst request address, and said second address, and wherein said tagcomparison block is further coupled to receive a plurality of tagaddresses from said cache, and wherein said tag comparison block isconfigured to compare said first request address to said plurality oftag addresses and to produce a first hit unpredicted way signal inresponse thereto, and wherein said tag comparison block is configured tocompare said second request address to said plurality of tag addressesand to produce a second hit unpredicted way signal in response thereto,and wherein said first hit unpredicted way signal indicates, whenasserted, that said first way prediction is incorrect and that saidfirst request address hits in a first way other than a first particularway indicated by said first way prediction, and wherein said second hitunpredicted way signal indicates, when asserted, that said second wayprediction is incorrect and that said second request address hits in asecond way other than a second particular way indicated by said secondway prediction; and a reservation station coupled to receive said firstand second hit unpredicted way signals from said tag comparison block,wherein said reservation is configured to invalidate a first datumforwarded in response to said first way prediction if said first hitunpredicted way signal is asserted, and wherein said reservation stationis configured to invalidate a second datum forwarded in response to saidsecond way prediction is said second hit unpredicted way signal isasserted.
 18. The superscalar microprocessor as recited in claim 17wherein said first way prediction comprises a first way value stored ina first selected one of said plurality of storage locations, and whereinsaid second way prediction comprises a second way value stored in asecond selected one of said plurality of storage locations.
 19. Thesuperscalar microprocessor as recited in claim 17 wherein said firstselected one of said plurality of storage locations is indexed by saidfirst request address, and wherein said second selected one of saidplurality of storage location is indexed by said second selected one ofsaid plurality of storage locations.